May 3, 2013 - He later became a leading proponent of eugenics, a movementânow discreditedâthat advocated improvement of the human ..... the answer...

13 downloads 6 Views 2MB Size

45

Pierce_5e_c03_045-076hr.indd 45

5/3/13 3:12 PM

46

Chapter 3

In 1993, scientists who were investigating a gene that affects the color of fur in mice discovered that the gene encodes the melanocortin-1 receptor. This receptor, when activated, increases the production of black eumelanin and decreases the production of red pheomelanin, resulting in black or brown fur. Shortly thereafter, the same melanocortin-1 receptor gene (MC1R) was located on human chromosome 16 and analyzed. When this gene is mutated in humans, red hair results. Most people with red hair carry two defective copies of the MC1R gene, which means that the trait is recessive (as originally proposed by the Davenports back in 1909). However, from 10% to 20% of redheads possess only a single mutant copy of MC1R, muddling the recessive interpretation of red hair (the people with a single mutant copy of the gene tend to have lighter red hair than those who harbor two mutant copies). The type and frequency of mutations at the MC1R gene vary widely among human populations, accounting for ethnic differences in the preponderance of red hair: among those of African and Asian descent, mutations for red hair are uncommon, whereas almost 40% of the people in the northern part of the United Kingdom carry at least one mutant copy of the gene for red hair. Modern humans are not the only people with red hair. Analysis of DNA from ancient bones indicates that some Neanderthals also carried a mutation in the MC1R gene that almost certainly caused red hair, but the mutation is distinct from those seen in modern humans.

T

his chapter is about the principles of heredity: how genes—such as the one for the melanocortin-1 receptor— are passed from generation to generation and how factors such as dominance influence that inheritance. The principles of heredity were first put forth by Gregor Mendel, and so we begin this chapter by examining Mendel’s scientific achievements. We then turn to simple genetic crosses, those in which a single characteristic is examined. We will consider some techniques for predicting the outcome of genetic crosses and then turn to crosses in which two or more characteristics are examined. We will see how the principles applied to simple genetic crosses and the ratios of offspring that they produce serve as the key for understanding more -complicated crosses. The chapter ends with a discussion of statistical tests for analyzing crosses. Throughout this chapter, a number of concepts are interwoven: Mendel’s principles of segregation and independent assortment, probability, and the behavior of chromosomes. These concepts might at first appear to be unrelated, but they are actually different views of the same phenomenon because the genes that undergo segregation and independent assortment are located on chromosomes. This chapter aims to examine these different views and to clarify their relations.

Mendel was born in what is now part of the Czech epublic. Although his parents were simple farmers with R little money, he received a sound education and was admitted to the Augustinian monastery in Brno in September 1843. After graduating from seminary, Mendel was ordained a priest and appointed to a teaching position in a local school. He excelled at teaching, and the abbot of the monastery recommended him for further study at the University of Vienna, which he attended from 1851 to 1853. There, Mendel enrolled

3.1 Gregor Mendel Discovered the Basic Principles of Heredity In 1909, when the Davenports speculated about the inheritance of red hair, the basic principles of heredity were just becoming widely known among biologists. Surprisingly, these principles had been discovered some 44 years earlier by Gregor Johann Mendel (1822–1884; Figure 3.1).

Pierce_5e_c03_045-076hr.indd 46

3.1 Gregor Johann Mendel, experimenting with peas, first discovered the principles of heredity. [James King-Holmes/Photo Researchers.]

5/3/13 3:12 PM

Basic Principles of Heredity

in the newly opened Physics Institute and took courses in mathematics, chemistry, entomology, paleontology, botany, and plant physiology. It was probably there that Mendel acquired knowledge of the scientific method, which he later applied so successfully to his genetics experiments. After two years of study in Vienna, Mendel returned to Brno, where he taught school and began his experimental work with pea plants. He conducted breeding experiments from 1856 to 1863 and presented his results publicly at meetings of the Brno Natural Science Society in 1865. Mendel’s paper from these lectures was published in 1866. However, in spite of widespread interest in heredity, the effect of his research on the scientific community was minimal. At the time, no one seemed to have noticed that Mendel had discovered the basic principles of inheritance. In 1868, Mendel was elected abbot of his monastery, and increasing administrative duties brought an end to his teaching and eventually to his genetics experiments. He died at the age of 61 on January 6, 1884, unrecognized for his contribution to genetics. The significance of Mendel’s discovery was not recognized until 1900, when three botanists—Hugo de Vries, Erich von Tschermak-Seysenegg, and Carl Correns—began independently conducting similar experiments with plants and arrived at conclusions similar to those of Mendel. Coming across Mendel’s paper, they interpreted their results in accord with his principles and drew attention to his pioneering work.

Mendel’s Success Mendel’s approach to the study of heredity was effective for several reasons. Foremost was his choice of experimental subject, the pea plant Pisum sativum (Figure 3.2), which offered clear advantages for genetic investigation. The plant is easy to cultivate, and Mendel had the monastery garden

Seed (endosperm) color

Yellow

Green

Pod color

Seed shape

Round Wrinkled

47

and greenhouse at his disposal. Compared with some other plants, peas grow relatively rapidly, completing an entire generation in a single growing season. By today’s standards, one generation per year seems frightfully slow—fruit flies complete a generation in 2 weeks and bacteria in 20 minutes— but Mendel was under no pressure to publish quickly and was able to follow the inheritance of individual characteristics for several generations. Had he chosen to work on an organism with a longer generation time—horses, for example— he might never have discovered the basis of inheritance. Pea plants also produce many offspring—their seeds—which allowed Mendel to detect meaningful mathematical ratios in the traits that he observed in the progeny. The large number of varieties of peas that were available to Mendel also was crucial because these varieties differed in various traits and were genetically pure. Mendel was therefore able to begin with plants of variable, known genetic makeup. Much of Mendel’s success can be attributed to the seven characteristics that he chose for study (see Figure 3.2). He avoided characteristics that display a range of variation; instead, he focused his attention on those that exist in two easily differentiated forms, such as white versus gray seed coats, round versus wrinkled seeds, and inflated versus constricted pods. Finally, Mendel was successful because he adopted an experimental approach and interpreted his results by using mathematics. Unlike many earlier investigators who just described the results of crosses, Mendel formulated hypotheses based on his initial observations and then conducted additional crosses to test his hypotheses. He kept careful records of the numbers of progeny possessing each type of trait and computed ratios of the different types. He was adept at seeing patterns in detail and was patient and thorough, conducting his experiments for 10 years before attempting to write up his results. Try Problem 13

Seed coat color

Gray

White

Flower position

Stem length

Axial (along stem)

Pod shape

Terminal (at tip of stem) Yellow

Green

Inflated

Constricted

Short

Tall

3.2 Mendel used the pea plant Pisum sativum in his studies of heredity. He examined seven characteristics that appeared in the seeds and in plants grown from the seeds. [Photograph by Charles Stirling/Alamy.]

Pierce_5e_c03_045-076hr.indd 47

5/3/13 3:12 PM

48

Chapter 3

CONCEPTS Gregor Mendel put forth the basic principles of inheritance, publishing his findings in 1866. Much of Mendel’s success can be attributed to the seven characteristics that he studied.

CONCEPT CHECK 1 Which of the following factors did not contribute to Mendel’s success in his study of heredity? a. His use of the pea plant b. His study of plant chromosomes c. His adoption of an experimental approach d. His use of mathematics

Genetic Terminology Before we examine Mendel’s crosses and the conclusions that he drew from them, a review of some terms commonly used in genetics will be helpful (Table 3.1). The term gene is a word that Mendel never knew. It was not coined until 1909, when Danish geneticist Wilhelm Johannsen first used it. The definition of a gene varies with the context of its use, and so its definition will change as we explore different aspects of heredity. For our present use in the context of genetic crosses, we will define a gene as an inherited factor that determines a characteristic. Genes frequently come in different versions called alleles (Figure 3.3). In Mendel’s crosses, seed shape was determined by a gene that exists as two different alleles: one allele encodes round seeds and the other encodes wrinkled seeds. All alleles for any particular gene will be found at a specific place

Table 3.1 Summary of important genetic terms Term Definition Gene An inherited factor (region of DNA) that helps determine a characteristic Allele One of two or more alternative forms of a gene Locus Specific place on a chromosome occupied by an allele Genotype Set of alleles possessed by an individual organism Heterozygote An individual organism possessing two different alleles at a locus Homozygote An individual organism possessing two of the same alleles at a locus Phenotype or trait The appearance or manifestation of a characteristic Characteristic or An attribute or feature possessed by an character organism

Pierce_5e_c03_045-076hr.indd 48

Genes exist in different versions called alleles.

One allele encodes round seeds…

Allele R

…and a different allele encodes wrinkled seeds.

Allele r Different alleles for a particular gene occupy the same locus on homologous chromosomes.

3.3 At each locus, a diploid organism possesses two alleles located on different homologous chromosomes. The alleles identified here refer to traits studied by Mendel.

on a chromosome called the locus for that gene. (The plural of locus is loci; it’s bad form in genetics—and incorrect—to speak of locuses.) Thus, there is a specific place—a locus—on a chromosome in pea plants where the shape of seeds is determined. This locus might be occupied by an allele for round seeds or one for wrinkled seeds. We will use the term allele when referring to a specific version of a gene; we will use the term gene to refer more generally to any allele at a locus. The genotype is the set of alleles that an individual organism possesses. A diploid organism with a genotype consisting of two identical alleles is homozygous for that locus. One that has a genotype consisting of two different alleles is heterozygous for the locus. Another important term is phenotype, which is the manifestation or appearance of a characteristic. A phenotype can refer to any type of characteristic—physical, physiological, biochemical, or behavioral. Thus, the condition of having round seeds is a phenotype, a body weight of 50 kilograms (50 kg) is a phenotype, and having sickle-cell anemia is a phenotype. In this book, the term characteristic or character refers to a general feature such as eye color; the term trait or phenotype refers to specific manifestations of that feature, such as blue or brown eyes. A given phenotype arises from a genotype that develops within a particular environment. The genotype determines the potential for development; it sets certain limits, or boundaries, on that development. How the phenotype develops within those limits is determined by the effects of other genes and of environmental factors, and the balance between these effects varies from characteristic to characteristic. For some characteristics, the differences between phenotypes are determined largely by differences in genotype. In Mendel’s peas, for example, the genotype, not the environment, largely determined the shape of the seeds. For other characteristics, environmental differences are more important. The height reached by an oak tree at maturity is a phenotype that is strongly influenced by environmental factors, such as the

5/3/13 3:12 PM

Basic Principles of Heredity

availability of water, sunlight, and nutrients. Nevertheless, the tree’s genotype still imposes some limits on its height: an oak tree will never grow to be 300 meters (almost 1000 feet) tall no matter how much sunlight, water, and fertilizer are provided. Thus, even the height of an oak tree is determined to some degree by genes. For many characteristics, both genes and environment are important in determining phenotypic differences. An obvious but important concept is that only the alleles of the genotype are inherited. Although the phenotype is determined, at least to some extent, by genotype, organisms do not transmit their phenotypes to the next generation. The distinction between genotype and phenotype is one of the most important principles of modern genetics. The next section describes Mendel’s careful observation of phenotypes through several generations of breeding experiments. These experiments allowed him to deduce not only the genotypes of the individual plants, but also the rules governing their inheritance.

49

Experiment Question: When peas with two different traits—round and wrinkled seeds—are crossed, will their progeny exhibit one of those traits, both of those traits, or an intermediate trait? Methods Stigma Anthers 1 To cross different varieties of peas, Mendel removed the anthers from flowers to prevent self-fertilization…

Flower

2 …and dusted the stigma with pollen from a different plant.

Cross

3 The pollen fertilized ova, which developed into seeds. 4 The seeds grew into plants.

CONCEPTS Each phenotype results from a genotype developing within a specific environment. The alleles of the genotype, not the phenotype, are inherited.

CONCEPT CHECK 2

P generation Homozygous Homozygous round seeds wrinkled seeds

What is the difference between a locus and an allele? What is the difference between genotype and phenotype? 5 Mendel crossed two homozygous varieties of peas.

Cross

3.2 Monohybrid Crosses Reveal the Principle of Segregation and the Concept of Dominance Mendel started with 34 varieties of peas and spent 2 years selecting those varieties that he would use in his experiments. He verified that each variety was pure-breeding (homozygous for each of the traits that he chose to study) by growing the plants for two generations and confirming that all offspring were the same as their parents. He then carried out a number of crosses between the different varieties. Although peas are normally self-fertilizing (each plant crosses with itself), Mendel conducted crosses between different plants by opening the buds before the anthers (male sex organs) were fully developed, removing the anthers, and then dusting the stigma (female sex organs) with pollen from a different plant’s anthers (Figure 3.4). Mendel began by studying monohybrid crosses—those between parents that differed in a single characteristic. In one experiment, Mendel crossed a pure-breeding (homozygous) pea plant for round seeds with one that was pure-breeding for wrinkled seeds (see Figure 3.4). This first generation of a cross is the P (parental) generation.

Pierce_5e_c03_045-076hr.indd 49

F1 generation

Selffertilize

6 All the F1 seeds were round. Mendel allowed plants grown from these seeds to selffertilize.

Results F2 generation

Fraction of progeny seeds 7

5474 round seeds

3/4 round

1850 wrinkled seeds

1/4 wrinkled

3.4 Mendel conducted monohybrid crosses.

5/3/13 3:12 PM

50

Chapter 3

After crossing the two varieties in the P generation, Mendel observed the offspring that resulted from the cross. In regard to seed characteristics, such as seed shape, the phenotype develops as soon as the seed matures because the seed traits are determined by the newly formed embryo within the seed. For characteristics associated with the plant itself, such as stem length, the phenotype doesn’t develop until the plant grows from the seed; for these characteristics, Mendel had to wait until the following spring, plant the seeds, and then observe the phenotypes on the plants that germinated. The offspring from the parents in the P generation are the F1 (first filial) generation. When Mendel examined the F1 generation of this cross, he found that they expressed only one of the phenotypes present in the parental generation: all the F1 seeds were round. Mendel carried out 60 such crosses and always obtained this result. He also conducted reciprocal crosses: in one cross, pollen (the male gamete) was taken from a plant with round seeds and, in its reciprocal cross, pollen was taken from a plant with wrinkled seeds. Reciprocal crosses gave the same result: all the F1 were round. Mendel wasn’t content with examining only the seeds arising from these monohybrid crosses. The following spring, he planted the F1 seeds, cultivated the plants that germinated from them, and allowed the plants to selffertilize, producing a second generation—the F2 (second filial) generation. Both of the traits from the P generation emerged in the F2 generation; Mendel counted 5474 round seeds and 1850 wrinkled seeds in the F2 (see Figure 3.4). He noticed that the number of the round and wrinkled seeds constituted approximately a 3 to 1 ratio; that is, about 3/4 of the F2 seeds were round and 1/4 were wrinkled. Mendel conducted monohybrid crosses for all seven of the characteristics that he studied in pea plants and, in all of the crosses, he obtained the same result: all of the F1 resembled only one of the two parents, but both parental traits emerged in the F2 in an approximate ratio of 3 : 1.

What Monohybrid Crosses Reveal First, Mendel reasoned that, although the F1 plants display the phenotype of only one parent, they must inherit genetic factors from both parents because they transmit both phenotypes to the F2 generation. The presence of both round and wrinkled seeds in the F2 plants could be explained only if the F1 plants possessed both round and wrinkled genetic factors that they had inherited from the P generation. He concluded that each plant must therefore possess two genetic factors encoding a characteristic. The genetic factors (now called alleles) that Mendel discovered are, by convention, designated with letters; the allele for round seeds is usually represented by R, and the allele for wrinkled seeds by r. The plants in the P generation of Mendel’s cross possessed two identical alleles: RR in the round-seeded parent and rr in the wrinkled-seeded parent (Figure 3.5a).

Pierce_5e_c03_045-076hr.indd 50

1 Mendel crossed a plant homozygous for round seeds (RR) with a plant homozygous for wrinkled seeds (rr).

(a) P generation

Homozygous round seeds

Homozygous wrinkled seeds

RR

rr

Gamete formation

Gamete formation

2 The two alleles in each plant separated when gametes were formed; one allele went into each gamete.

r

Gametes

R

Fertilization

(b) F1 generation

Round seeds

3 Gametes fused to produce heterozygous F1 plants that had round seeds because round is dominant over wrinkled.

Rr Gamete formation

R r

4 Mendel self-fertilized the F1 to produce the F2,…

R r

Gametes

Self–fertilization

(c) F2 generation

Round

Round

Wrinkled

3/4 round 1/4 wrinkled

5 …which appeared in a 3 1 ratio of round to wrinkled.

1/4 Rr

1/4 RR

1/4 rR

1/4 rr

Gamete formation

Gametes R 6 Mendel also selffertilized the F2,…

R

R

r

r

R

r

r

Self–fertilization

(d) F3 generation 7 …to produce F3 seeds.

Round Round

RR

Wrinkled Wrinkled Round

RR

rr

rr

Rr rR Homozygous round peas produced plants with only round peas.

Heterozygous plants produced round and wrinkled seeds in a 3 1 ratio.

Homozygous wrinkled peas produced plants with only wrinkled peas.

3.5 Mendel’s monohybrid crosses revealed the principle of segregation and the concept of dominance.

5/3/13 3:12 PM

The second conclusion that Mendel drew from his monohybrid crosses was that the two alleles in each plant separate when gametes are formed, and one allele goes into each gamete. When two gametes (one from each parent) fuse to produce a zygote, the allele from the male parent unites with the allele from the female parent to produce the genotype of the offspring. Thus, Mendel’s F1 plants inherited an R allele from the round-seeded plant and an r allele from the wrinkled-seeded plant (Figure 3.5b). However, only the trait encoded by the round allele (R) was observed in the F1: all the F1 progeny had round seeds. Those traits that appeared unchanged in the F1 heterozygous offspring Mendel called dominant, and those traits that disappeared in the F1 heterozygous offspring he called recessive. Alleles for dominant traits are often symbolized with uppercase letters (e.g. R), while alleles for recessive traits are often symbolized with lowercase letters (e.g. r). When dominant and recessive alleles are present together, the recessive allele is masked, or suppressed. The concept of dominance was the third important conclusion that Mendel derived from his monohybrid crosses. Mendel’s fourth conclusion was that the two alleles of an individual plant separate with equal probability into the gametes. When plants of the F1 (with genotype Rr) produced gametes, half of the gametes received the R allele for round seeds and half received the r allele for wrinkled seeds. The gametes then paired randomly to produce the following genotypes in equal proportions among the F2: RR, Rr, rR, rr (Figure 3.5c). Because round (R) is dominant over wrinkled (r), there were three round progeny in the F2 (RR, Rr, rR) for every one wrinkled progeny (rr) in the F2. This 3 : 1 ratio of round to wrinkled progeny that Mendel observed in the F2 could be obtained only if the two alleles of a genotype separated into the gametes with equal probability. The conclusions that Mendel developed about inheritance from his monohybrid crosses have been further developed and formalized into the principle of segregation and the concept of dominance. The principle of segregation (Mendel’s first law, see Table 3.2) states that each individual diploid organism possesses two alleles for any particular characteristic, one inherited from the maternal parent and one from the paternal parent. These two alleles segregate (separate) when gametes are formed, and one allele goes into each gamete. Furthermore, the two alleles segregate into gametes in equal proportions. The concept of dominance states that, when two different alleles are present in a genotype, only the trait encoded by one of them—the “dominant” allele—is observed in the phenotype. Mendel confirmed these principles by allowing his F2 plants to self-fertilize and produce an F3 generation. He found that the plants grown from the wrinkled seeds—those displaying the recessive trait (rr)—produced an F3 in which all plants produced wrinkled seeds. Because his wrinkledseeded plants were homozygous for wrinkled alleles (rr), only wrinkled alleles could be passed on to their progeny (Figure 3.5d).

Pierce_5e_c03_045-076hr.indd 51

Basic Principles of Heredity

51

Table 3.2 Comparison of the principles of segregation and independent assortment Observation Principle

Stage of Meiosis*

Segregation 1. (Mendel’s first law) 2.

Each individual Before meiosis organism possesses two alleles encoding a trait. Alleles separate Anaphase I when gametes are formed. 3. Alleles separate in Anaphase I equal proportions.

Independent Alleles at different Anaphase I assortment loci separate independently. (Mendel’s second law) *Assumes that no crossing over occurs. If crossing over takes place, then segregation and independent assortment may also occur in anaphase II of meiosis.

The plants grown from round seeds—the dominant trait— fell into two types (see Figure 3.5c). On self-fertilization, about 2/3 of these plants produced both round and wrinkled seeds in the F3 generation. These plants were heterozygous (Rr); so they produced 1/4 RR (round), 1/2 Rr (round), and 1/4 rr (wrinkled) seeds, giving a 3 : 1 ratio of round to wrinkled in the F3 . About 1/3 of the plants grown from round seeds were of the second type; they produced only the round-seeded trait in the F3 . These plants were homozygous for the round allele (RR) and could thus produce only round offspring in the F3 generation. Mendel planted the seeds obtained in the F3 and carried these plants through three more rounds of self-fertilization. In each generation, 2 /3 of the round-seeded plants produced round and wrinkled offspring, whereas 1/3 produced only round offspring. These results are entirely consistent with the principle of segregation. CONCEPTS The principle of segregation states that each individual organism possesses two alleles that can encode a characteristic. These alleles segregate when gametes are formed, and one allele goes into each gamete. The concept of dominance states that, when the two alleles of a genotype are different, only the trait encoded by one of them—the “dominant” allele—is observed.

CONCEPT CHECK 3 How did Mendel know that each of his pea plants carried two alleles encoding a characteristic?

5/3/13 3:12 PM

52

Chapter 3

CONNECTING CONCEPTS Relating Genetic Crosses to Meiosis We have now seen how the results of monohybrid crosses are explained by Mendel’s principle of segregation. Many students find that they enjoy working genetic crosses but are frustrated by the abstract nature of the symbols. Perhaps you feel the same at this point. You may be asking, “What do these symbols really represent? What does the genotype RR mean in regard to the biology of the organism?” The answers to these questions lie in relating the abstract symbols of crosses to the structure and behavior of chromosomes, the repositories of genetic information (see Chapter 2).

In 1900, when Mendel’s work was rediscovered and biologists began to apply his principles of heredity, the relation between genes and chromosomes was still unclear. The theory that genes are located on chromosomes (the chromosome theory of heredity) was developed in the early 1900s by Walter Sutton, then a graduate student at Columbia University. Through the careful study of meiosis in insects, Sutton documented the fact that each homologous pair of chromosomes consists of one maternal chromosome and one paternal chromosome. Showing that these pairs

(a) 1 The two alleles of genotype Rr are located on homologous chromosomes,…

R

r

Chromosome replication

2 …which replicate in the S phase of meiosis.

R

Rr

r

3 In prophase I of meiosis, crossing over may or may not take place. Prophase I No crossing over (b)

Crossing over (c)

R

Rr

r

R

rR

r

4 In anaphase I, the homologous chromosomes separate. Anaphase I

R

R

r

Anaphase II

R

Anaphase I

R

5 If no crossing over has taken place, the two chromatids of each chromosome segregate in anaphase II and are identical.

r

Anaphase II

r

r

R

6 If crossing over has taken place, the two chromatids are no longer identical, and the different alleles segregate in anaphase II.

r

R

Anaphase II

R

r

r

Anaphase II

R

r

3.6 Segregation results from the separation of homologous chromosomes in meiosis.

Pierce_5e_c03_045-076hr.indd 52

5/3/13 3:12 PM

Basic Principles of Heredity

segregate independently into gametes in meiosis, he concluded that this process is the biological basis for Mendel’s principles of heredity. German cytologist and embryologist Theodor Boveri came to similar conclusions at about the same time. The symbols used in genetic crosses, such as R and r, are just shorthand notations for particular sequences of DNA in the chromosomes that encode particular phenotypes. The two alleles of a genotype are found on different but homologous chromosomes. One chromosome of each homologous pair is inherited from the mother and the other is inherited from the father. In the S phase of meiotic interphase, each chromosome replicates, producing two copies of each allele, one on each chromatid (Figure 3.6a). The homologous chromosomes segregate in anaphase I, thereby separating the two different alleles (Figure 3.6b and c). This chromosome segregation is the basis of the principle of segregation. In anaphase II of meiosis, the two chromatids of each replicated chromosome separate; so each gamete resulting from meiosis carries only a single allele at each locus, as Mendel’s principle of segregation predicts. If crossing over has taken place in prophase I of meiosis, then the two chromatids of each replicated chromosome are no longer identical, and the segregation of different alleles takes place at anaphase I and anaphase II (see Figure 3.6c). However, Mendel didn’t know anything about chromosomes; he formulated his principles of heredity entirely on the basis of the results of the crosses that he carried out. Nevertheless, we should not forget that these principles work because they are based on the behavior of actual chromosomes in meiosis. Try Problem 30

The Molecular Nature of Alleles Let’s take a moment to consider in more detail exactly what an allele is and how it determines a phenotype. Although Mendel had no information about the physical nature of the genetic factors in his crosses, modern geneticists have now determined the molecular basis of these factors and how they encode a trait like wrinkled peas. Alleles, such as R and r that code for round and wrinkled peas, usually represent specific DNA sequences. The locus that determines whether a pea is round or wrinkled is a sequence of DNA on pea chromosome 5 that encodes a protein called starch-branching enzyme isoform 1 (SBEI). The R allele, which produces round seeds in pea plants, codes for a normal, functional form of the SBEI enzyme. This enzyme converts a linear form of starch into a highly branched form. The r allele, which encodes wrinkled seeds, is a different DNA sequence that contains a mutation or error; it encodes a nonactive form of the enzyme that does not produce the branched form of starch and leads to the accumulation of sucrose within the rr pea. Because the rr pea contains a large amount of sucrose, the developing seed absorbs water and swells. Later, as the pea matures, it loses water. Because rr peas absorbed more water and expanded more during development, they lose more water during maturation and afterwards appear shriveled or wrinkled. The r allele for wrinkled seeds is recessive because the presence of a single R allele in the heterozygote encodes enough SBEI e nzyme to produce branched starch and round seeds.

Pierce_5e_c03_045-076hr1.indd 53

53

Research has revealed that the r allele contains an extra 800 base pairs of DNA that disrupt the normal coding sequence of the gene. The extra DNA appears to have come from a transposable element, a type of DNA sequence that has the ability to move from one location in the genome to another, which we will discuss further in Chapter 18.

Predicting the Outcomes of Genetic Crosses One of Mendel’s goals in conducting his experiments on pea plants was to develop a way to predict the outcome of crosses between plants with different phenotypes. In this section, you will first learn a simple, shorthand method for predicting outcomes of genetic crosses (the Punnett square), and then you will learn how to use probability to predict the results of crosses. THE PUNNETT SQUARE The Punnett square was devel-

oped by English geneticist Reginald C. Punnett in 1917. To illustrate the Punnett square, let’s examine another cross carried out by Mendel. By crossing two varieties of peas that differed in height, Mendel established that tall (T) was dominant over short (t). He tested his theory concerning the inheritance of dominant traits by crossing an F1 tall plant that was heterozygous (Tt) with the short homozygous parental variety (tt). This type of cross, between an F1 genotype and either of the parental genotypes, is called a backcross. To predict the types of offspring that result from this backcross, we first determine which gametes will be produced by each parent (Figure 3.7a). The principle of segregation tells us that the two alleles in each parent separate, and one allele passes to each gamete. All gametes from the homozygous tt short plant will receive a single short (t) allele. The tall plant in this cross is heterozygous (Tt); so 50% of its gametes will receive a tall allele (T) and the other 50% will receive a short allele (t). A Punnett square is constructed by drawing a grid, putting the gametes produced by one parent along the upper edge and the gametes produced by the other parent down the left side (Figure 3.7b). Each cell (a block within the Punnett square) contains an allele from each of the corresponding gametes, generating the genotype of the progeny produced by fusion of those gametes. In the upper left-hand cell of the Punnett square in Figure 3.7b, a gamete containing T from the tall plant unites with a gamete containing t from the short plant, giving the genotype of the progeny (Tt). It is useful to write the phenotype expressed by each genotype; here the progeny will be tall, because the tall allele is dominant over the short allele. This process is repeated for all the cells in the Punnett square. By simply counting, we can determine the types of progeny produced and their ratios. In Figure 3.7b, two cells contain tall (Tt) progeny and two cells contain short (tt) progeny; so the genotypic ratio expected for this cross is 2 Tt to 2 tt (a 1 : 1 ratio). Another way to express this result is to say that we expect 1/2 of the progeny to have genotype Tt (and phenotype tall) and 1/2 of the progeny to have genotype tt (and phenotype short). In this cross, the genotypic ratio and

6/10/13 11:37 AM

54

Chapter 3

PROBABILITY AS A TOOL OF GENETICS Another

(a) P generation

Tall

Short

Tt

tt

Gametes T t

t t

Fertilization (b) F1 generation

t

t

Tt

Tt

Tall

Tall

tt

tt

Short

Short

T

t

ethod for determining the outcome of a genetic cross is to m use the rules of probability, as Mendel did with his crosses. Probability expresses the likelihood of the occurrence of a particular event. It is the number of times that a particular event takes place, divided by the number of all possible outcomes. For example, a deck of 52 cards contains only one king of hearts. The probability of drawing one card from the deck at random and obtaining the king of hearts is 1/52 because there is only one card that is the king of hearts (one event) and there are 52 cards that can be drawn from the deck (52 possible outcomes). The probability of drawing a card and obtaining an ace is 4/52 because there are four cards that are aces (four events) and 52 cards (possible outcomes). Probability can be expressed either as a fraction (4/52 in this case) or as a decimal number (0.077 in this case). The probability of a particular event may be determined by knowing something about how or how often the event takes place. We know, for example, that the probability of rolling a six-sided die and getting a four is 1/6 because the die has six sides and any one side is equally likely to end up on top. So, in this case, understanding the nature of the event—the shape of the thrown die—allows us to determine the probability. In other cases, we determine the probability of an event by making a large number of observations. When a weather forecaster says that there is a 40% chance of rain on a particular day, this probability was obtained by observing a large number of days with similar atmospheric conditions and finding that it rains on 40% of those days. In this case, the probability has been determined empirically (by observation). THE MULTIPLICATION RULE Two rules of probability

Conclusion: Genotypic ratio Phenotypic ratio

. 1 Tt . 1 tt . 1 tall . 1 short

3.7 The Punnett square can be used to determine the results of a genetic cross.

the phenotypic ratio are the same, but this outcome need not be the case. Try completing a Punnett square for the cross in which the F1 round-seeded plants in Figure 3.5 undergo selffertilization (you should obtain a phenotypic ratio of 3 round to 1 wrinkled and a genotypic ratio of 1 RR to 2 Rr to 1 rr). CONCEPTS The Punnett square is a shorthand method of predicting the genotypic and phenotypic ratios of progeny from a genetic cross.

CONCEPT CHECK 4 If an F1 plant depicted in Figure 3.5 is backcrossed to the parent with round seeds, what proportion of the progeny will have wrinkled seeds? (Use a Punnett square.) 1 a. 3/4 c. /4 1 b. /2 d. 0

Pierce_5e_c03_045-076hr1.indd 54

are useful for predicting the ratios of offspring produced in genetic crosses. The first is the multiplication rule, which states that the probability of two or more independent events taking place together is calculated by multiplying their independent probabilities. To illustrate the use of the multiplication rule, let’s again consider the roll of a die. The probability of rolling one die and obtaining a four is 1/6 . To calculate the probability of rolling a die twice and obtaining 2 fours, we can apply the multiplication rule. The probability of obtaining a four on the first roll is 1/6 and the probability of obtaining a four on the second roll is 1/6 ; so the probability of rolling a four on both is 1/6 Ž 1/6 = 1/36 (Figure 3.8a). The key indicator for applying the multiplication rule is the word and; in the example just considered, we wanted to know the probability of obtaining a four on the first roll and a four on the second roll. For the multiplication rule to be valid, the events whose joint probability is being calculated must be independent— the outcome of one event must not influence the outcome of the other. For example, the number that comes up on one roll of the die has no influence on the number that comes up on the other roll; so these events are independent. However, if we wanted to know the probability of being hit on the head with a hammer and going to the hospital on the same day, we

6/10/13 11:37 AM

Basic Principles of Heredity

could not simply apply the multiplication rule and multiply the two events together, because they are not independent— being hit on the head with a hammer certainly influences the probability of going to the hospital. THE ADDITION RULE The second rule of probability fre-

quently used in genetics is the addition rule, which states that the probability of any one of two or more mutually exclusive events is calculated by adding the probabilities of these events. Let’s look at this rule in concrete terms. To obtain the probability of throwing a die once and rolling either a three or a four, we would use the addition rule, adding the probability of obtaining a three (1/6) to the probability of obtaining a four (again, 1/6), or 1/6 + 1/6 = 2/6 = 1/3 (Figure 3.8b). The key indicators for applying the addition rule are the words either and or. For the addition rule to be valid, the events whose probability is being calculated must be mutually exclusive, meaning that one event excludes the possibility of the occurrence of the other event. For example, you cannot throw a single die just once and obtain both a three and a four, because only one side of the die can be on top. These events are mutually exclusive.

55

(a) The multiplication rule

1 If you roll a die,… 2 …in a large number of sample rolls, on average, one out of six times you will obtain a four;…

Roll 1

3 …so the probability of obtaining a four in any roll is 1/6. 4 If you roll the die again,… 5 …your probability of getting four is again 1/6;…

Roll 2

CONCEPTS The multiplication rule states that the probability of two or more independent events taking place together is calculated by multiplying their independent probabilities. The addition rule states that the probability that any one of two or more mutually exclusive events taking place is calculated by adding their probabilities.

CONCEPT CHECK 5 If the probability of being blood-type A is 1/8 and the probability of blood-type O is 1/2, what is the probability of being either blood-type A or blood-type O? 1 a. 5/8 c. /10 1 1 b. /2 d. /16

APPLying PROBABILITY TO GENETIC CROSSES The

multiplication and addition rules of probability can be used in place of the Punnett square to predict the ratios of progeny expected from a genetic cross. Let’s first consider a cross between two pea plants heterozygous for the locus that determines height, Tt Ž Tt. Half of the gametes produced by each plant have a T allele, and the other half have a t allele; so the probability for each type of gamete is 1/2 . The gametes from the two parents can combine in four different ways to produce offspring. Using the multiplication rule, we can determine the probability of each possible type. To calculate the probability of obtaining TT progeny, for example, we multiply the probability of receiving a T allele from the first parent (1/2) times the probability of receiving a T allele from the second parent (1/2). The multiplication rule should be used here because we need the probability of receiving a T allele from the first parent and a T allele from

Pierce_5e_c03_045-076hr.indd 55

6

…so the probability of getting a four on the first roll and the second roll is 1/6 1/6 = 1/36 .

(b) The addition rule 1 If you roll a die,… 2 …on average, one out of six times you'll get a three… 3 …and one out of six times you'll get a four.

4 That is, the probability of getting either a three or a four is 1/6 + 1/6 = 2/6 = 1/3. 3.8 The multiplication and addition rules can be used to determine the probability of combinations of events.

the second parent—two independent events. The four types of progeny from this cross and their associated probabilities are: TT (T gamete and T gamete)

1

/2 Ž 1/2 = 1/4 tall

Tt (T gamete and t gamete)

1

tT (t gamete and T gamete)

1

tt (t gamete and t gamete)

1

/2 Ž 1/2 = 1/4 tall /2 Ž 1/2 = 1/4 tall /2 Ž 1/2 = 1/4 short

5/3/13 3:12 PM

56

Chapter 3

Notice that there are two ways for heterozygous progeny to be produced: a heterozygote can either receive a T allele from the first parent and a t allele from the second or receive a t allele from the first parent and a T allele from the second. After determining the probabilities of obtaining each type of progeny, we can use the addition rule to determine the overall phenotypic ratios. Because of dominance, a tall plant can have genotype TT, Tt, or tT; so, using the addition rule, we find the probability of tall progeny to be 1/4 + 1/4 + 1/4 = 3 /4 . Because only one genotype encodes short (tt), the probability of short progeny is simply 1/4 . Two methods have now been introduced to solve genetic crosses: the Punnett square and the probability method. At this point, you may be asking, “Why bother with probability rules and calculations? The Punnett square is easier to understand and just as quick.” This is true for simple monohybrid crosses. However, for tackling more-complex crosses concerning genes at two or more loci, the probability method is both clearer and quicker than the Punnett square. CONDITIONAL PROBABILITY Thus far, we have used

probability to predict the chances of producing certain types of progeny given only the genotypes of the parents. Sometimes we have additional information that modifies or “conditions” the probability, a situation termed conditional probability. For example, assume that we cross two heterozygous pea plants (Tt Ž Tt) and obtain a tall progeny. What is the probability that this tall plant is heterozygous (Tt)? You might assume that the probability would be 1/2 , the probability of obtaining a heterozygous progeny in a cross between two heterozygotes. However, in this case we have some additional information— the phenotype of the progeny plant—which modifies the probability. When two heterozygous individuals are crossed, we expect 1/4 TT, 1/2 Tt, and 1/4 tt progeny. We know that the progeny in question is tall, so we can eliminate the possibility that it has genotype tt. Tall progeny must be either genotype TT or genotype Tt and, in a cross between two heterozygotes, these occur in a 1 : 2 ratio. Therefore, the probability that a tall progeny is heterozygous (Tt) is two out of three, or 2/3 . THE BINOMIAL EXPANSION AND PROBABILITY When

probability is used, it is important to recognize that there may be several different ways in which a set of events can occur. Consider two parents who are both heterozygous for albinism, a recessive condition in humans that causes reduced pigmentation in the skin, hair, and eyes (Figure 3.9; see also the introduction to Chapter 1). When two parents heterozygous for albinism mate (Aa Ž Aa), the probability of their having a child with albinism (aa) is 1/4 and the probability of having a child with normal pigmentation (AA or Aa) is 3/4 . Suppose we want to know the probability of this couple having three children with albinism. In this case, there is only one way in which this can happen: their first child has albinism and their second child has albinism and their third child has albinism. Here, we simply apply the multiplication rule: 1/4 Ž 1/4 Ž 1/4 = 1/64 .

Pierce_5e_c03_045-076hr1.indd 56

3.9 Albinism in human beings is usually inherited as a recessive trait. [Richard Dranitzke/SS/Photo Researchers.]

Suppose we now ask what the probability is of this couple having three children, one with albinism and two with normal pigmentation? This situation is more complicated. The first child might have albinism, whereas the second and third are unaffected; the probability of this sequence of events is 1 /4 Ž 3/4 Ž 3/4 = 9/64 . Alternatively, the first and third children might have normal pigmentation, whereas the second has albinism; the probability of this sequence is 3/4 Ž 1/4 Ž 3/4 = 9/64 . Finally, the first two children might have normal pigmentation and the third albinism; the probability of this sequence is 3/4 Ž 3/4 Ž 1/4 = 9/64 . Because either the first sequence or the second sequence or the third sequence produces one child with albinism and two with normal pigmentation, we apply the addition rule and add the probabilities: 9 /64 + 9/64 + 9/64 = 27/64 . If we want to know the probability of this couple having five children, two with albinism and three with normal pigmentation, figuring out all the different combinations of children and their probabilities becomes more difficult. This task is made easier if we apply the binomial expansion. The binomial takes the form ( p + q)n, where p equals the probability of one event, q equals the probability of the alternative event, and n equals the number of times the event occurs. For figuring the probability of two out of five children with albinism: p = the probability of a child having albinism (1/4) q = the probability of a child having normal pigmentation (3/4) The binomial for this situation is ( p + q)5 because there are five children in the family (n = 5). The expansion is: ( p + q)5 = p5 + 5p4q + 10p3q2 + 10p2q3 + 5pq4 + q5

6/10/13 11:37 AM

Basic Principles of Heredity

Each of the terms in the expansion provides the probability for one particular combination of traits in the children. The first term in the expansion ( p5) equals the probability of having five children all with albinism, because p is the probability of albinism. The second term (5p4q) equals the probability of having four children with albinism and one with normal pigmentation, the third term (10p3q2) equals the probability of having three children with albinism and two with normal pigmentation, and so forth. To obtain the probability of any combination of events, we insert the values of p and q; so the probability of having two out of five children with albinism is: 10 p2q3 = 10 (1/4)2 (3/4)3 = 270/1024 = 0.26 We could easily figure out the probability of any desired combination of albinism and pigmentation among five children by using the other terms in the expansion. How did we expand the binomial in this example? In general, the expansion of any binomial ( p + q)n consists of a series of n + 1 terms. In the preceding example, n = 5; so there are 5 + 1 = 6 terms: p5, 5p4q, 10p3q2, 10p2q3, 5pq4, and q5. To write out the terms, first figure out their exponents. The exponent of p in the first term always begins with the power to which the binomial is raised, or n. In our example, n equals 5, so our first term is p5. The exponent of p decreases by one in each successive term; so the exponent of p is 4 in the second term ( p4), 3 in the third term ( p3), and so forth. The exponent of q is 0 (no q) in the first term and increases by 1 in each successive term, increasing from 0 to 5 in our example. Next, determine the coefficient of each term. The coefficient of the first term is always 1; so, in our example, the first term is 1p5, or just p5. The coefficient of the second term is always the same as the power to which the binomial is raised; in our example, this coefficient is 5 and the term is 5p4q. For the coefficient of the third term, look back at the preceding term; multiply the coefficient of the preceding term (5 in our example) by the exponent of p in that term (4) and then divide by the number of that term (second term, or 2). So the coefficient of the third term in our example is (5 Ž 4)/2 = 20/2 = 10 and the term is 10p3q2. Follow this procedure for each successive term. The coefficients for the terms in the binomial expansion can also be determined from Pascal’s triangle (Table 3.3). The exponents and coefficients for each term in the first 5 binomial expansions are given in Table 3.4. Another way to determine the probability of any particular combination of events is to use the following formula: n! s t P= pq s!t! where P equals the overall probability of event X with probability p occurring s times and event Y with probability q occurring t times. For our albinism example, event X would be the occurrence of a child with albinism (1/4) and event Y would be the occurrence of a child with normal pigmentation

Pierce_5e_c03_045-076hr.indd 57

57

Table 3.3 Pascal’s Triangle The numbers on each row represent the coefficients of each term in the binomial expansion ( p + q)n.

n Coefficients 1 1

1 1

2

1 2 1

3

1 3 3 1

4

1 4 6 4 1

5

1 5 10 10 5 1

6

1 6 15 20 15 6 1

Note: Each number in the triangle, except for the 1s, is equal to the sum of the two numbers directly above it.

Table 3.4 Coefficients and Terms for the

Binomial Expansion (p + q)n for n = 1 through 5

n

Binomial expansion

1

a+b

2

a2 + 2ab + b2

3

a3 + 3a2b + 3ab2 + b3

4

a4 + 4a3b + 6a2b2 + 4ab3 + b4

5

a5 + 5a4b + 10a3b2 + 10a2b3 + 5ab4 + b5

(3/4); s would equal the number of children with albinism (2) and t would equal the number of children with normal pigmentation (3). The ! symbol stands for factorial, and it means the product of all the integers from n to 1. In this example, n = 5; so n! = 5 Ž 4 Ž 3 Ž 2 Ž 1. Applying this formula to obtain the probability of two out of five children having albinism, we obtain: P= =

5! 1 2 3 3 1 / 2 1 /4 2 2!3! 4

5Ž4Ž3Ž2Ž1 1 2 3 3 1 / 2 1 /4 2 = 0.26 2Ž1Ž3Ž2Ž1 4

This value is the same as that obtained with the binomial expansion. Try Problems 25, 26, AND 27

The Testcross A useful tool for analyzing genetic crosses is the testcross, in which one individual of unknown genotype is crossed with another individual with a homozygous recessive genotype for the trait in question. Figure 3.7 illustrates a testcross (in this case, it is also a backcross). A testcross tests, or reveals, the genotype of the first individual. Suppose you were given a tall pea plant with no information about its parents. Because tallness is a dominant trait in

5/3/13 3:12 PM

58

Chapter 3

peas, your plant could be either homozygous (TT) or heterozygous (Tt), but you would not know which. You could determine its genotype by performing a testcross. If the plant were homozygous (TT), a testcross would produce all tall progeny (TT Ž tt S all Tt); if the plant were heterozygous (Tt), half of the progeny would be tall and half would be short (Tt Ž tt S 1/2 Tt and 1/2 tt). When a testcross is performed, any recessive allele in the unknown genotype is expressed in the progeny, because it will be paired with a recessive allele from the homozygous recessive parent. Try Problems 18 AND 21 CONCEPTS The binomial expansion can be used to determine the probability of a particular set of events. A testcross is a cross between an individual with an unknown genotype and one with a homozygous recessive genotype. The outcome of the testcross can reveal the unknown genotype.

Genetic Symbols As we have seen, genetic crosses are usually depicted with the use of symbols to designate the different alleles. The symbols used for alleles are usually determined by the community of geneticists who work on a particular organism and therefore there is no universal system for designating symbols. In plants, lowercase letters are often used to designate recessive alleles, and uppercase letters are for dominant alleles. Two or three letters may be used for a single allele: the recessive allele for heart-shaped leaves in cucumbers is designated hl, and the recessive allele for abnormal sperm-head shape in mice is designated azh. In animals, the common allele for a character—called the wild type because it is the allele usually found in the wild—is often symbolized by one or more letters and a plus sign (+). The letter or letters chosen are usually based on the mutant (unusual) phenotype. For example, the recessive allele for yellow eyes in the Oriental fruit fly is represented by ye, whereas the allele for wild-type eye color is represented by ye+. At times, the letters for the wild-type allele are dropped and the allele is represented simply by a plus sign. Superscripts and subscripts are sometimes added to distinguish between genes: Lfr1 and Lfr2 represent dominant mutant alleles at different loci that produce lacerate leaf margins in opium poppies; El R represents an allele in goats that restricts the length of the ears. A slash may be used to distinguish alleles present in an individual genotype. For example, the genotype of a goat that is heterozygous for restricted ears might be written El+/El R or simply +/El R. If genotypes at more than one locus are presented together, a space separates the genotypes. For example, a goat heterozygous for a pair of alleles that produces restricted ears and heterozygous for another pair of alleles that produces goiter can be designated by El +/El R G /g. Sometimes it is useful to designate the possibility of several genotypes. A line in a genotype, such as A_, indicates that any allele is possible. In this case, A_ might include both AA and Aa genotypes.

Pierce_5e_c03_045-076hr.indd 58

CONNECTING CONCEPTS Ratios in Simple Crosses Now that we have had some experience with genetic crosses, let’s review the ratios that appear in the progeny of simple crosses, in which a single locus is under consideration and one of the alleles is dominant over the other. Understanding these ratios and the parental genotypes that produce them will enable you to work simple genetic crosses quickly, without resorting to the Punnett square. Later, we will use these ratios to work more-complicated crosses that include several loci. There are only three phenotypic ratios to understand (Table 3.5). The 3 : 1 ratio arises in a simple genetic cross when both of the parents are heterozygous for a dominant trait (Aa Ž Aa). The second phenotypic ratio is the 1 : 1 ratio which results from the mating of a heterozygous parent and a homozygous parent. The homozygous parent in this cross must carry two recessive alleles (Aa Ž aa) to obtain a 1 : 1 ratio, because a cross between a homozygous dominant parent and a heterozygous parent (AA Ž Aa) produces offspring displaying only the dominant trait. The third phenotypic ratio is not really a ratio: all the offspring have the same phenotype (uniform progeny). Several combinations of parents can produce this outcome (see Table 3.5). A cross between any two homozygous parents—either between two of the same homozygotes (AA Ž AA or aa Ž aa) or between two different homozygotes (AA Ž aa)—produces progeny all having the same phenotype. Progeny of a single phenotype also can result from a cross between a homozygous dominant parent and a heterozygote (AA Ž Aa). If we are interested in the ratios of genotypes instead of phenotypes, there are only three outcomes to remember (Table 3.6): the

Table 3.5 Phenotypic ratios for simple genetic crosses (crosses for a single locus) with dominance Phenotypic Ratio

Genotypes of Parents

Genotypes of Progeny

3 : 1

Aa Ž Aa

3

1 : 1

Aa Ž aa

1

Uniform progeny

AA Ž AA All AA

aa Ž aa All aa

AA Ž aa All Aa

AA Ž Aa All A_

/4 A_ : 1/4 aa /2 Aa : 1/2 aa

Table 3.6 Genotypic ratios for simple genetic crosses (crosses for a single locus) Genotypic Ratio

Genotypes of Parents

Genotypes of Progeny

1 : 2 : 1

Aa Ž Aa

1

1 : 1

Aa Ž aa

1

Aa Ž AA

1

Uniform progeny

AA Ž AA All AA

aa Ž aa All aa

AA Ž aa All Aa

/4 AA : 1/2 Aa : 1/4 aa /2 Aa : 1/2 aa /2 Aa : 1/2 AA

5/3/13 3:12 PM

Basic Principles of Heredity

1 : 2 : 1 ratio, produced by a cross between two heterozygotes; the 1 : 1 ratio, produced by a cross between a heterozygote and a homozygote; and the uniform progeny produced by a cross between two homozygotes. These simple phenotypic and genotypic ratios and the parental genotypes that produce them provide the key to understanding crosses for a single locus and, as you will see in the next section, for multiple loci.

Experiment Question: Do alleles encoding different traits separate independently? (a) Methods

3.3 Dihybrid Crosses Reveal the Principle of Independent Assortment

P generation Round, yellow seeds

RR YY

ry Fertilization

(b)

In addition to his work on monohybrid crosses, Mendel crossed varieties of peas that differed in two characteristics—a dihybrid cross. For example, he had one homozygous variety of pea with seeds that were round and yellow; another homozygous variety with seeds that were wrinkled and green. When he crossed the two varieties, the seeds of all the F1 progeny were round and yellow. He then self-fertilized the F1 and obtained the following progeny in the F2 : 315 round, yellow seeds; 101 wrinkled, yellow seeds; 108 round, green seeds; and 32 wrinkled, green seeds. Mendel recognized that these traits appeared approximately in a 9 : 3 : 3 : 1 ratio; that is, 9/16 of the progeny were round and yellow, 3/16 were wrinkled and yellow, 3 /16 were round and green, and 1/16 were wrinkled and green.

Pierce_5e_c03_045-076hr.indd 59

rr yy

Gametes RY

Dihybrid Crosses

Mendel carried out a number of dihybrid crosses for pairs of characteristics and always obtained a 9 : 3 : 3 : 1 ratio in the F2 . This ratio makes perfect sense in regard to segregation and dominance if we add a third principle, which Mendel recognized in his dihybrid crosses: the principle of independent assortment (Mendel’s second law). This principle states that alleles at different loci separate independently of one another (see Table 3.2). A common mistake is to think that the principle of segregation and the principle of independent assortment refer to two different processes. The principle of independent assortment is really an extension of the principle of segregation. The principle of segregation states that the two alleles of a locus separate when gametes are formed; the principle of independent assortment states that, when these two alleles separate, their separation is independent of the separation of alleles at other loci. Let’s see how the principle of independent assortment explains the results that Mendel obtained in his dihybrid cross. Each plant possesses two alleles encoding each characteristic, and so the parental plants must have had genotypes RR YY and rr yy (Figure 3.10a). The principle of segregation

Wrinkled, green seeds

We will now extend Mendel’s principle of segregation to more-complex crosses that include alleles at multiple loci. Understanding the nature of these crosses will require an additional principle, the principle of independent assortment.

The Principle of Independent Assortment

59

F1 generation

Round, yellow seeds

Rr Yy

Gametes RY

ry

Ry

rY

Self–fertilization (c) Results

F2 generation

RY

ry

Ry

rY

RR YY

Rr Yy

RR Yy

Rr YY

Rr Yy

rr yy

Rr yy

rr Yy

RR Yy

Rr yy

RR yy

Rr Yy

Rr YY

rr Yy

Rr Yy

rr YY

RY

ry

Ry

rY

Phenotypic ratio 9 round, yellow 3 round, green 3 wrinkled, yellow 1 wrinkled, green Conclusion: The allele encoding color separated independently of the allele encoding seed shape, producing a 9 3 3 1 ratio in the F2 progeny. 3.10 Mendel’s dihybrid crosses revealed the principle of independent assortment.

5/3/13 3:12 PM

60

Chapter 3

indicates that the alleles for each locus separate, and one allele for each locus passes to each gamete. The gametes produced by the round, yellow parent therefore contain alleles RY, whereas the gametes produced by the wrinkled, green parent contain alleles ry. These two types of gametes unite to produce the F1, all with genotype Rr Yy. Because round is dominant over wrinkled and yellow is dominant over green, the phenotype of the F1 will be round and yellow. When Mendel self-fertilized the F1 plants to produce the F2, the alleles for each locus separated, with one allele going into each gamete. This event is where the principle of independent assortment becomes important. Each pair of alleles can separate in two ways: (1) R separates with Y, and r separates with y, to produce gametes RY and ry or (2) R separates with y, and r separates with Y, to produce gametes Ry and rY. The principle of independent assortment tells us that the alleles at each locus separate independently; thus, both kinds R

1 This cell contains two pairs of homologous chromosomes.

of separation take place equally and all four types of gametes (RY, ry, Ry, and rY) are produced in equal p roportions (Figure 3.10b). When these four types of gametes are combined to produce the F2 generation, the progeny consist of 9/16 round and yellow, 3/16 wrinkled and yellow, 3/16 round and green, and 1/16 wrinkled and green, resulting in a 9 : 3 : 3 : 1 phenotypic ratio (Figure 3.10c).

Relating the Principle of Independent Assortment to Meiosis An important qualification of the principle of independent assortment is that it applies to characteristics encoded by loci located on different chromosomes because, like the principle of segregation, it is based wholly on the behavior of chromosomes in meiosis. Each pair of homologous chromosomes separates independently of all other pairs in anaphase I of meiosis (Figure 3.11); so genes located on different pairs of r

Y

3.11 The principle of independent assortment results from the independent separation of chromosomes in anaphase I of meiosis.

y

Chromosome replication

RR

2 In anaphase I of meiosis, each pair of homologous chromosomes separates independently;…

RR

rr

YY

Anaphase II

R

Y

R

rr

YY y y

Anaphase I

RR

yy

Anaphase II

Y

r

y

r

rr

yy

Anaphase II

y

R

y

R

YY

Anaphase II

y

r

Y

r

Y

3 …so genes located on different pairs of chromosomes assort independently, producing different combinations of alleles in the gametes.

Pierce_5e_c03_045-076hr.indd 60

5/3/13 3:12 PM

Basic Principles of Heredity

homologs will assort independently. Genes that happen to be located on the same chromosome will travel together during anaphase I of meiosis and will arrive at the same destination— within the same gamete (unless crossing over takes place). However, genes located on the same chromosome do not assort independently (unless they are located sufficiently far apart that crossing over takes place in every meiotic division, as will be discussed fully in Chapter 7). CONCEPTS The principle of independent assortment states that genes encoding different characteristics separate independently of one another when gametes are formed, owing to the independent separation of homologous pairs of chromosomes in meiosis. Genes located close together on the same chromosome do not, however, assort independently.

CONCEPT CHECK 6 How are the principles of segregation and independent assortment related and how are they different?

Applying Probability and the Branch Diagram to Dihybrid Crosses When the genes at two loci separate independently, a dihybrid cross can be understood as two monohybrid crosses. Let’s examine Mendel’s dihybrid cross (Rr Yy Ž Rr Yy) by considering each characteristic separately (Figure 3.12a). If we consider only the shape of the seeds, the cross was Rr Ž Rr, which yields a 3 : 1 phenotypic ratio (3/4 round and 1/4 wrinkled progeny, see Table 3.5). Next consider the other characteristic, the color of the seed. The cross was Yy Ž Yy, which produces a 3 : 1 phenotypic ratio (3/4 yellow and 1/4 green progeny). We can now combine these monohybrid ratios by using the multiplication rule to obtain the proportion of progeny with different combinations of seed shape and color. The proportion of progeny with round and yellow seeds is 3/4 (the probability of round) Ž 3/4 (the probability of yellow) = 9/16 . The proportion of progeny with round and green seeds is 3 /4 Ž 1/4 = 3/16 ; the proportion of progeny with wrinkled and yellow seeds is 1/4 Ž 3/4 = 3/16 ; and the proportion of progeny with wrinkled and green seeds is 1/4 Ž 1/4 = 1/16 . Branch diagrams are a convenient way of organizing all the combinations of characteristics (Figure 3.12b). In the first column, list the proportions of the phenotypes for one c haracter (here, 3/4 round and 1/4 wrinkled). In the second column, list the proportions of the phenotypes for the second character (3/4 yellow and 1/4 green) twice, next to each of the phenotypes in the first column: put 3/4 yellow and 1/4 green next to the round phenotype and again next to the wrinkled phenotype. Draw lines between the phenotypes in the first column and each of the phenotypes in the second column. Now follow each branch of the diagram, multiplying the probabilities for each trait along that branch. One branch leads from round to yellow, yielding round and yellow progeny. Another branch leads from round to green, yielding round and green progeny,

Pierce_5e_c03_045-076hr.indd 61

Round, yellow

61

Round, yellow

Rr Yy

Rr Yy

1 The dihybrid cross is broken into two monohybrid crosses…

(a)

Expected proportions for first character (shape)

Expected proportions for second character (color)

Expected proportions for both characters

Rr Rr

Yy Yy

Rr Yy Rr Yy

Cross

Cross

3/4

R_

Round 1/4

Yellow

yy

Green

3 The individual characters and the associated probabilities are then combined by using the branch method.

(b)

3/4

3/4 Y_ 1/4

rr

Wrinkled

2 …and the probability of each character is determined.

R_

3/4 Y_

R_ Y_

Yellow

3/4

1/4

R_ yy

Round

yy

Green

3/4 = 9/16 Round, yellow

3/4

1/4 = 3/16 Round, green

3/4 Y_

rr Y_

Yellow

1/4

1/4

rr yy

1/4 rr

3/4 = 3/16 Wrinkled, yellow

Wrinkled

yy

Green

1/4

1/4 = 1/16 Wrinkled, green

3.12 A branch diagram can be used to determine the phenotypes and expected proportions of offspring from a dihybrid cross (Rr Yy Ž Rr Yy).

and so forth. We calculate the probability of progeny with a particular combination of traits by using the multiplication rule: the probability of round (3/4) and yellow (3/4) seeds is 3 /4 Ž 3/4 = 9/16 . The advantage of the branch diagram is that it helps keep track of all the potential combinations of traits that may appear in the progeny. It can be used to determine phenotypic or genotypic ratios for any number of characteristics. Using probability is much faster than using the Punnett square for crosses that include multiple loci. Genotypic and phenotypic ratios can be quickly worked out by combining, with the multiplication rule, the simple ratios in Tables 3.5 and 3.6. The probability method is particularly efficient if we need the probability of only a particular phenotype or genotype among the progeny of a cross. Suppose that we need to know the probability of obtaining the genotype Rr yy in the F2 of

5/3/13 3:12 PM

62

Chapter 3

the dihybrid cross in Figure 3.10. The probability of obtaining the Rr genotype in a cross of Rr Ž Rr is 1/2 and that of obtaining yy progeny in a cross of Yy Ž Yy is 1/4 (see Table 3.6). Using the multiplication rule, we find the probability of Rr yy to be 1/2 Ž 1/4 = 1/8 . To illustrate the advantage of the probability method, consider the cross Aa Bb cc Dd Ee Ž Aa Bb Cc dd Ee. Suppose that we want to know the probability of obtaining offspring with the genotype aa bb cc dd ee. If we use a Punnett square to determine this probability, we might be working on the solution for months. However, we can quickly figure the probability of obtaining this one genotype by breaking this cross into a series of single-locus crosses:

Progeny cross

Probability

aa

1

Bb Ž Bb

bb

1

cc Ž Cc

cc

1

Aa Ž Aa

A @

Genotype

/4

Round, yellow

Rr Yy

1/2

Rr rr

Yy yy

Cross

Cross

Rr

Round 1/2

rr

Wrinkled

1

Ee Ž Ee

ee

1

/2

1/2

The Dihybrid Testcross Let’s practice using the branch diagram by determining the types and proportions of phenotypes in a dihybrid testcross between the round and yellow F1 plants (Rr Yy) obtained by Mendel in his dihybrid cross and the wrinkled and green plants (rr yy), as depicted in Figure 3.13. Break the cross down into a series of single-locus crosses. The cross Rr Ž rr yields 1/2 round (Rr) progeny and 1/2 wrinkled (rr) progeny. The cross Yy Ž yy yields 1/2 yellow (Yy) progeny and 1 /2 green (yy) progeny. Using the multiplication rule, we find the proportion of round and yellow progeny to be 1/2 (the probability of round) Ž 1/2 (the probability of yellow) = 1/4 . Four combinations of traits with the following proportions appear in the offspring: 1/4 Rr Yy, round yellow; 1/4 Rr yy, round green; 1/4 rr Yy, wrinkled yellow; and 1/4 rr yy, wrinkled green.

Pierce_5e_c03_045-076hr.indd 62

Rr Yy rr yy

Yy

Yellow 1/2

yy

Green

Yy

Rr Yy

Yellow

1/2

1/2

Rr yy

Rr

1/2 = 1/4 Round, yellow

1/2

yy

Green

1/2

1/2

rr Yy

Yy

1/2 = 1/4 Round, green

Yellow

1/2

1/2

rr yy

rr

1/2 = 1/4 Wrinkled, yellow

Wrinkled

yy

Green

CONCEPTS A cross including several characteristics can be worked by breaking the cross down into single-locus crosses and using the multiplication rule to determine the proportions of combinations of characteristics (provided that the genes assort independently).

Expected proportions for both characters

Round

/4

The probability of an offspring from this cross having genotype aa bb cc dd ee is now easily obtained by using the multiplication rule: 1/4 Ž 1/4 Ž 1/2 Ž 1/2 Ž 1/4 = 1/256 . This calculation assumes that genes at these five loci all assort independently. Now that you’ve had some experience working genetic crosses, explore Mendel’s principles of heredity by setting up some of your own crosses in Animation 3.1.

1/2

1/2

/2

dd

rr yy

Expected Expected proportions for proportions for first character second character

/4

Dd Ž dd

Wrinkled, green

1/2

1/2 = 1/4 Wrinkled, green

3.13 A branch diagram can be used to determine the phenotypes and expected proportions of offspring from a dihybrid testcross (Rr Yy Ž rr yy).

WORKED PROBLEM

The principles of segregation and independent assortment are important not only because they explain how heredity works, but also because they provide the means for predicting the outcome of genetic crosses. This predictive power has made genetics a powerful tool in agriculture and other fields, and the ability to apply the principles of heredity is an important skill for all students of genetics. Practice with genetic problems is essential for mastering the basic principles of heredity; no amount of reading and memorization can substitute for the experience gained by deriving solutions to specific problems in genetics. You may find genetics problems difficult if you are unsure of where to begin or how to organize a solution to the problem. In genetics, every problem is different, so no common series of steps can be applied to all genetics problems. Logic and common sense must be used to analyze a problem and arrive at a solution. Nevertheless, certain steps can facilitate

5/3/13 3:12 PM

Basic Principles of Heredity

the process, and solving the following problem will serve to illustrate these steps. In mice, black coat color (B) is dominant over brown (b), and a solid pattern (S) is dominant over white spotted (s). Color and spotting are controlled by genes that assort independently. A homozygous black, spotted mouse is crossed with a homozygous brown, solid mouse. All the F1 mice are black and solid. A testcross is then carried out by mating the F1 mice with brown, spotted mice. a. Give the genotypes of the parents and the F1 mice. b. Give the genotypes and phenotypes, along with their expected ratios, of the progeny expected from the testcross. Solution Strategy What information is required in your answer to the problem?

First, determine what question or questions the problem is asking. Is it asking for genotypes, genotypic ratios, or phenotypic ratios? This problem asks you to provide the genotypes of the parents and the F1, the expected genotypes and phenotypes of the progeny of the testcross, and their expected proportions. What information is provided to solve the problem?

Next determine what information is provided that will be necessary to solve the problem. This problem gives important information about the dominance relations of the characters and the genes that code for the traits. ■■ black is dominant over brown. ■■ solid is dominant over white spotted. ■■ the genes for the two characteristics assort independently. ■■ symbols for the different alleles: B for black, b for brown, S for solid, and s for spotted. It is often helpful to write down the symbols at the beginning of the solution: B—black S—solid b—brown s—white spotted Next, write out the crosses given in the problem. P

Homozygous black, spotted

Homozygous brown, solid

Black, solid

F1 Testcross

Ž

Black, solid

Ž

If you need help solving the problem, review those sections of the chapter that cover the relevant information. For this problem, review Sections 3.2 and 3.3.

Pierce_5e_c03_045-076hr1.indd 63

Solution Steps STEP 1 Write down any genetic information that can be determined from the phenotypes alone.

From the phenotypes and the statement that they are homozygous, you know that the P-generation mice must be BB ss and bb SS. The F1 mice are black and solid, both dominant traits, and so the F1 mice must possess at least one black allele (B) and one solid allele (S). At this point, you cannot be certain about the other alleles; so represent the genotype of the F1 as B_ S_ , where _ means that any allele is possible. The brown, spotted mice in the testcross must be bb ss because both brown and spotted are recessive traits that will be expressed only if two recessive alleles are present. Record these genotypes on the crosses that you wrote out in step 2: P

Homozygous black, spotted BB ss

Testcross

Ž

Homozygous brown, solid bb SS

Black, solid B_ S_

F1 Black, solid B_ S_

Ž

Brown, spotted bb ss

STEP 2 Break the problem down into smaller parts.

First, determine the genotype of the F1. After this genotype has been determined, you can predict the results of the testcross and determine the genotypes and phenotypes of the progeny from the testcross. Second, because this cross includes two independently assorting loci, it can be conveniently broken down into two single-locus crosses: one for coat color and the other for spotting. Third, use a branch diagram to determine the proportion of progeny of the testcross with different combinations of the two traits. STEP 3 Work the different parts of the problem.

Start by determining the genotype of the F1 progeny. Mendel’s first law indicates that the two alleles at a locus separate, one going into each gamete. Thus, the gametes produced by the black, spotted parent contain B s and the gametes produced by the brown, solid parent contain b S, which combine to produce F1 progeny with the genotype Bb Ss: P

Brown, spotted

For help with this problem, review:

63

Gametes F1

Homozygous black, spotted BB ss

Ž

Homozygous brown, solid bb SS

Bs bS 5 Bb Ss

6/4/13 3:57 PM

64

Chapter 3

Use the F1 genotype to work the testcross (Bb Ss Ž bb ss), breaking it into two single-locus crosses. First, consider the cross for coat color: Bb Ž bb. Any cross between a heterozygote and a homozygous recessive genotype produces a 1 : 1 phenotypic ratio of progeny (see Table 3.5): 887n

Bb Ž bb 1

/2 Bb black /2 bb brown

1

Next, do the cross for spotting: Ss Ž ss. This cross also is between a heterozygote and a homozygous recessive genotype and will produce 1/2 solid (Ss) and 1/2 spotted (ss) progeny (see Table 3.5).

1

887n

Ss Ž ss /2 Ss solid /2 ss spotted

1

Finally, determine the proportions of progeny with combinations of these characters by using the branch diagram. n /2 Bb black 8888n

1

/2 Ss solid

88n

Bb Ss black, solid 1 Ž 1 /2 /2 = 1/4

1

/2 ss spotted

88n

Bb ss black, spotted 1 /2 Ž 1/2 = 1/4

1

/2 Ss solid

88n

bb Ss brown, solid 1 Ž 1 /2 /2 = 1/4

1

88n

bb ss brown, spotted 1 /2 Ž 1/2 = 1/4

1

n /2 bb brown 8888n

1

/2 ss spotted

STEP 4 Check all work.

As a last step, reread the problem, checking to see if your answers are consistent with the information provided. You have used the genotypes BB ss and bb SS in the P generation. Do these genotypes encode the phenotypes given in the problem? Are the F1 progeny phenotypes consistent with the genotypes that you assigned? The answers are consistent with the information. Now that we have stepped through a genetics problem together try your hand at Problem 33 at the end of this chapter.

3.4 Observed Ratios of Progeny May Deviate from Expected Ratios by Chance When two individual organisms of known genotype are crossed, we expect certain ratios of genotypes and phenotypes in the progeny; these expected ratios are based on the

Pierce_5e_c03_045-076hr3.indd 64

Mendelian principles of segregation, independent assortment, and dominance. The ratios of genotypes and phenotypes actually observed among the progeny, however, may deviate from these expectations. For example, in German cockroaches, brown body color (Y) is dominant over yellow body color (y). If we cross a brown, heterozygous cockroach (Yy) with a yellow cockroach (yy), we expect a 1 : 1 ratio of brown (Yy) and yellow (yy) progeny. Among 40 progeny, we therefore expect to see 20 brown and 20 yellow offspring. However, the observed numbers might deviate from these expected values; we might in fact see 22 brown and 18 yellow progeny. Chance plays a critical role in genetic crosses, just as it does in flipping a coin. When you flip a coin, you expect a 1 : 1 ratio—1/2 heads and 1/2 tails. If you flip a coin 1000 times, the proportion of heads and tails obtained will probably be very close to that expected 1 : 1 ratio. However, if you flipped the coin 10 times, the ratio of heads to tails might be quite different from 1 : 1. You could easily get 6 heads and 4 tails, or 3 heads and 7 tails, just by chance. You might even get 10 heads and 0 tails. The same thing happens in genetic crosses. We may expect 20 brown and 20 yellow cockroaches, but 22 brown and 18 yellow progeny could arise as a result of chance.

The Chi-Square Goodness-of-Fit Test If you expected a 1 : 1 ratio of brown and yellow cockroaches but the cross produced 22 brown and 18 yellow, you probably wouldn’t be too surprised even though it wasn’t a perfect 1 : 1 ratio. In this case, it seems reasonable to assume that chance produced the deviation between the expected and the observed results. But, if you observed 25 brown and 15 yellow, would you still assume that this represents a 1 : 1 ratio? Something other than chance might have caused the deviation. Perhaps the inheritance of this characteristic is more complicated than was assumed or perhaps some of the yellow progeny died before they were counted. Clearly, we need some means of e valuating how likely it is that chance is responsible for the deviation between the observed and the expected numbers. To evaluate the role of chance in producing deviations between observed and expected values, a statistical test called the chi-square goodness-of-fit test is used. This test provides information about how well observed values fit expected values. Before we learn how to calculate the chi square, it is important to understand what this test does and does not indicate about a genetic cross. The chi-square test cannot tell us whether a genetic cross has been correctly carried out, whether the results are correct, or whether we have chosen the correct genetic explanation for the results. What it does indicate is the probability that the difference between the observed and the expected values is due to chance. In other words, it indicates the likelihood that chance alone could produce the deviation between the expected and the observed values. If we expected 20 brown and 20 yellow progeny from a genetic cross, the chi-square test gives the probability that we might observe 25 brown and 15 yellow progeny simply owing to chance deviations from the expected 20 : 20 ratio. This

6/18/13 3:24 PM

Basic Principles of Heredity

hypothesis, that chance alone is responsible for any deviations between observed and expected values, is sometimes called the null hypothesis. Statistics such as the chi-square test cannot prove that the null hypothesis is correct, but they can help us decide whether we should reject it. When the probability calculated from the chi-square test is high, we assume that chance alone produced the difference and we do not reject the null hypothesis. When the probability is low, we assume that some factor other than chance—some significant factor—produced the deviation. For example, the mortality of the yellow cockroaches might be higher than that of brown cockroaches. When the probability that chance produced the deviation is low, we reject the null hypothesis (the null hypothesis is false). To use the chi-square goodness-of-fit test, we first determine the expected results. The chi-square test must always be applied to numbers of progeny, not to proportions or percentages. Let’s consider a locus for coat color in domestic cats, for which black color (B) is dominant over gray (b). If we crossed two heterozygous black cats (Bb Ž Bb), we would expect a 3 : 1 ratio of black and gray kittens. A series of such crosses yields a total of 50 kittens—30 black and 20 gray. These numbers are our observed values. We can obtain the expected numbers by multiplying the expected proportions by the total number of observed progeny. In this case, the expected number of black kittens is 3/4 Ž 50 = 37.5 and the expected number of gray kittens is 1/4 Ž 50 = 12.5. The chi-square (2) value is calculated by using the following formula: 2 =

1 observed - expected 2 2 expected

65

in which means the sum. We calculate the sum of all the squared differences between observed and expected and divide by the expected values. To calculate the chi-square value for our black and gray kittens, we first subtract the number of expected black kittens from the number of observed black kittens (30 - 37.5 = -7.5) and square this value: -7.52 = 56.25. We then divide this result by the expected number of black kittens, 56.25/37.5 = 1.5. We repeat the calculations on the number of expected gray kittens: (20 - 12.5)2/12.5 = 4.5. To obtain the overall chi-square value, we sum the (observed - expected)2/expected values: 1.5 + 4.5 = 6.0. The next step is to determine the probability associated with this calculated chi-square value, which is the probability that the deviation between the observed and the expected results could be due to chance. This step requires us to compare the calculated chi-square value (6.0) with theoretical values that have the same degrees of freedom in a chi-square table. The degrees of freedom represent the number of ways in which the expected classes are free to vary. For a goodnessof-fit chi-square test, the degrees of freedom are equal to n - 1, in which n is the number of different expected phenotypes. Here, we lose one degree of freedom because the total number of expected progeny must equal the total number of observed progeny. In our example, there are two expected phenotypes (black and gray); so n = 2, and the degree of freedom equals 2 - 1 = 1. Now that we have our calculated chi-square value and have figured out the associated degrees of freedom, we are ready to obtain the probability from a chi-square table (Table 3.7). The degrees of freedom are given in the

Table 3.7 Critical values of the 2 distribution P df 0.995

0.975 0.9 0.5 0.1 0.05* 0.025 0.01 0.005

1 0.000

0.000 0.016 0.455 2.706 3.841 5.024 6.635 7.879

2 0.010

0.051 0.211 1.386 4.605 5.991 7.378 9.210 10.597

3 0.072

0.216 0.584 2.366 6.251 7.815 9.348 11.345 12.838

4 0.207

0.484 1.064 3.357 7.779 9.488 11.143 13.277 14.860

5 0.412

0.831 1.610 4.351 9.236 11.070 12.832 15.086 16.750

6 0.676

1.237 2.204 5.348 10.645 12.592 14.449 16.812 18.548

7 0.989

1.690 2.833 6.346 12.017 14.067 16.013 18.475 20.278

8 1.344

2.180 3.490 7.344 13.362 15.507 17.535 20.090 21.955

9 1.735

2.700 4.168 8.343 14.684 16.919 19.023 21.666 23.589

10 2.156

3.247 4.865 9.342 15.987 18.307 20.483 23.209 25.188

11 2.603

3.816 5.578 10.341 17.275 19.675 21.920 24.725 26.757

12 3.074

4.404 6.304 11.340 18.549 21.026 23.337 26.217 28.300

13 3.565

5.009 7.042 12.340 19.812 22.362 24.736 27.688 29.819

14 4.075

5.629 7.790 13.339 21.064 23.685 26.119 29.141 31.319

15 4.601

6.262 8.547 14.339 22.307 24.996 27.488 30.578 32.801

P, probability; df, degrees of freedom. *Most scientists assume that, when P 0.05, a significant difference exists between the observed and the expected values in a chi-square test

Pierce_5e_c03_045-076hr.indd 65

5/3/13 3:12 PM

66

Chapter 3

left-hand column of the table and the probabilities are given at the top; within the body of the table are chi-square values associated with these probabilities. First, find the row for the appropriate degrees of freedom; for our example with 1 degree of freedom, it is the first row of the table. Find where our calculated chi-square value (6.0) lies among the theoretical values in this row. The theoretical chi-square values increase from left to right and the probabilities decrease from left to right. Our chi-square value of 6.0 falls between the value of 5.024, associated with a probability of 0.025, and the value of 6.635, associated with a probability of 0.01. Thus, the probability associated with our chi-square value is less than 0.025 and greater than 0.01. So there is less than a 2.5% probability that the deviation that we observed between the expected and the observed numbers of black and gray kittens could be due to chance. Most scientists use the 0.05 probability level as their cutoff value: if the probability of chance being responsible for the deviation is greater than or equal to 0.05, they accept that chance may be responsible for the deviation between the observed and the expected values. When the probability is less than 0.05, scientists assume that chance is not responsible and a significant difference exists. The expression significant difference means that a factor other than chance is responsible for the observed values being different from the expected values. In regard to the kittens, perhaps one of the genotypes had a greater mortality rate before the progeny were counted or perhaps other genetic factors skewed the observed ratios. In choosing 0.05 as the cutoff value, scientists have agreed to assume that chance is responsible for the deviations between observed and expected values unless there is strong evidence to the contrary. Bear in mind that, even if we obtain a probability of, say, 0.01, there is still a 1% probability that the deviation between the observed and the expected numbers is due to nothing more than chance. Calculation of the chi-square value is illustrated in Figure 3.14. Try Problem 38

CONCEPTS Differences between observed and expected ratios can arise by chance. The chi-square goodness-of-fit test can be used to evaluate whether deviations between observed and expected numbers are likely to be due to chance or to some other significant factor.

CONCEPT CHECK 7 A chi-square test comparing observed and expected progeny is carried out, and the probability associated with the calculated chi-square value is 0.72. What does this probability represent?

Pierce_5e_c03_045-076hr.indd 66

P generation Purple flowers

White flowers

Cross

F1 generation

A plant with purple flowers is crossed with a plant with white flowers, and the F1 are self-fertilized…

Purple flowers Self-fertilize F2 generation

…to produce 105 F2 progeny with purple flowers and 45 with white flowers (an apparent 3 :1 ratio).

105 purple 45 white

Phenotype

Observed

Expected

Purple

105

3/4 150

White Total

45 150

1/4 150

2 = 2 = 2

=

2 =

= 37.5 The expected values are obtained by multiplying the expected proportion by the total,…

(O – E)2 E

(105– 112.5)2 112.5

= 112.5

+

(45– 37.5)2 37.5

56.25 112.5

+

56.25 37.5

0.5

+

…and then the chi-square value is calculated.

1.5 = 2.0

Degrees of freedom = n – 1 Degrees of freedom = 2– 1=1 Probability (from Table 3.5) 0.1 < P < 0.5

The probability associated with the calculated chi-square value is between 0.10 and 0.50, indicating a high probability that the difference between observed and expected values is due to chance.

Conclusion: No significant difference between observed and expected values. 3.14 A chi-square test is used to determine the probability that the difference between observed and expected values is due to chance.

a. Probability that the correct results were obtained b. Probability of obtaining the observed numbers c. Probability that the difference between observed and expected numbers is significant d. Probability that the difference between observed and expected numbers could be due to chance

5/3/13 3:12 PM

Basic Principles of Heredity

67

CONCEPTS SUMMARY

Gregor Mendel discovered the principles of heredity. His success can be attributed to his choice of the pea plant as an experimental organism, the use of characters with a few easily distinguishable phenotypes, his experimental approach, the use of mathematics to interpret his results, and careful attention to detail. ■■ Genes are inherited factors that determine a characteristic. Alternative forms of a gene are called alleles. The alleles are located at a specific place, a locus, on a chromosome, and the set of genes that an individual organism possesses is its genotype. Phenotype is the manifestation or appearance of a characteristic and may refer to a physical, biochemical, or behavioral characteristic. Only the genotype—not the phenotype—is inherited. ■■ The principle of segregation states that a diploid individual organism possesses two alleles encoding a trait and that these two alleles separate in equal proportions when gametes are formed. ■■

The concept of dominance indicates that, when two ifferent alleles are present in a heterozygote, only the trait d of one of them, the dominant allele, is observed in the phenotype. The other allele is said to be recessive. ■■ The two alleles of a genotype are located on homologous chromosomes. The separation of homologous chromosomes in anaphase I of meiosis brings about the segregation of alleles. ■■

Probability is the likelihood that a particular event will occur. The multiplication rule states that the probability of two or more independent events occurring together is calculated by multiplying the probabilities of the independent events. The addition rule states that the probability of any of two or more mutually exclusive events occurring is calculated by adding the probabilities of the events. ■■ The binomial expansion can be used to determine the probability of a particular combination of events. ■■ A testcross reveals the genotype (homozygote or heterozygote) of an individual organism having a dominant trait and consists of crossing that individual with one having the homozygous recessive genotype. ■■ The principle of independent assortment states that genes encoding different characteristics assort independently when gametes are formed. Independent assortment is based on the random separation of homologous pairs of chromosomes in anaphase I of meiosis; it takes place when genes encoding two characteristics are located on different pairs of chromosomes. ■■ Observed ratios of progeny from a genetic cross may deviate from the expected ratios owing to chance. The chi-square goodness-of-fit test can be used to determine the probability that a difference between observed and expected numbers is due to chance. ■■

IMPORTANT TERMS

gene (p. 48) allele (p. 48) locus (p. 48) genotype (p. 48) homozygous (p. 48) heterozygous (p. 48) phenotype (p. 48) monohybrid cross (p. 49) P (parental) generation (p. 49)

F1 (first filial) generation (p. 50) reciprocal cross (p. 50) F2 (second filial) generation (p. 50) dominant (p. 51) recessive (p. 51) principle of segregation (Mendel’s first law) (p. 51)

concept of dominance (p. 51) chromosome theory of heredity (p. 52) backcross (p. 53) Punnett square (p. 53) probability (p. 54) multiplication rule (p. 54) addition rule (p. 55)

conditional probability (p. 56) testcross (p. 57) wild type (p. 58) dihybrid cross (p. 59) principle of independent assortment (Mendel’s second law) (p. 59) chi-square goodness-of-fit test (p. 64)

ANSWERS TO CONCEPT CHECKS

1. b 2. A locus is a place on a chromosome where genetic information encoding a characteristic is located. An allele is a version of a gene that encodes a specific trait. A genotype is the set of alleles possessed by an individual organism, and a phenotype is the manifestation or appearance of a characteristic. 3. Because the traits for both alleles appeared in the F2 progeny

Pierce_5e_c03_045-076hr.indd 67

4. d 5. a 6. Both the principle of segregation and the principle of independent assortment refer to the separation of alleles in anaphase I of meiosis. The principle of segregation says that these alleles separate, and the principle of independent assortment says that they separate independently of alleles at other loci. 7. d

5/3/13 3:12 PM

68

Chapter 3

WORKED PROBLEMS Problem 1

Short hair (S) in rabbits is dominant over long hair (s). The following crosses are carried out, producing the progeny shown. Give all possible genotypes of the parents in each cross. Parents a. short Ž short

Progeny 4 short and 2 long

b. short Ž short

8 short

c. short Ž long

12 short

d. short Ž long

3 short and 1 long

e. long Ž long

2 long

Solution Strategy What information is required in your answer to the problem?

All possible genotypes of the parents in each cross. What information is provided to solve the problem? ■ ■ ■

Short hair is dominant over long hair. Phenotypes of the parents of each cross. Phenotypes and number of progeny of each cross.

For help with this problem, review:

Connecting Concepts: Ratios in Simple Crosses, in Section 3.2.

were heterozygous, we would expect 1/4 of the progeny to be long haired (ss), but we do not observe any long-haired progeny. The other parent could be homozygous (SS) or heterozygous (Ss); as long as one parent is homozygous, all the offspring will be short haired. It is theoretically possible, although unlikely, that both parents are heterozygous (Ss Ž Ss). If both were heterozygous, we would expect two of the eight progeny to be long haired. Although no long-haired progeny are observed, it is possible that just by chance no long-haired rabbits would be produced among the eight progeny of the cross. c. short Ž long

Solution Steps Note: The problem asks for all possible genotypes of the parents.

For this problem, it is useful to first gather as much information about the genotypes of the parents as possible on the basis of their phenotypes. We can then look at the types of progeny produced to provide the missing information.

a. short Ž short Note: When both parents are heterozygotes (Ss x Ss) we expect to see a 3:1 ratio in progeny, but just by chance the progeny exhibited a 4 : 2 ratio.

4 short and 2 long

Because short hair is dominant over long hair, a rabbit having short hair could be either SS or Ss. The 2 long-haired offspring must be homozygous (ss) because long hair is recessive and will appear in the phenotype only when both alleles for long hair are present. Because each parent contributes one of the two alleles found in the progeny, each parent must be carrying the s allele and must therefore be Ss. b. short Ž short

8 short

The short-haired parents could be SS or Ss. All eight of the offspring are short (S_), and so at least one of the parents is likely to be homozygous (SS); if both parents

Pierce_5e_c03_045-076hr1.indd 68

12 short

The short-haired parent could be SS or Ss. The long-haired parent must be ss. If the short-haired parent were heterozygous (Ss), half of the offspring would be expected to be long haired, but we don’t see any long-haired progeny. Therefore, this parent is most likely homozygous (SS). It is theoretically possible, although unlikely, that the parent is heterozygous and just by chance no long-haired progeny were produced. d. short Ž long

3 short and 1 long

On the basis of its phenotype, the short-haired parent could be homozygous (SS) or heterozygous (Ss), but the presence of one long-haired offspring tells us that the short-haired parent must be heterozygous (Ss). The long-haired parent must be homozygous (ss). e. long Ž long

2 long

Because long hair is recessive, both parents must be homozygous for a long-hair allele (ss).

6/4/13 3:59 PM

Basic Principles of Heredity

69

Problem 2

In cats, black coat color is dominant over gray. A female black cat whose mother is gray mates with a gray male. If this female has a litter of six kittens, what is the probability that three will be black and three will be gray? Solution Strategy What information is required in your answer to the problem?

What information is provided to solve the problem? ■

Black is dominant to gray.

■

The mother of the litter is black and her mother is gray.

■

The father of the litter is gray.

For help with this problem, review:

The binomial expansion and probability, in Section 3.2. Solution Steps

Because black (G) is dominant over gray (g), a black cat may be homozygous (GG) or heterozygous (Gg). The black female in this problem must be heterozygous (Gg) because her mother is gray (gg) and she must inherit one of her mother’s alleles. The gray male is homozygous (gg)

Hint: We can determine the female parent’s genotype from her phenotype and her mother’s phenotype.

Gg Ž gg Black female Gray male 887n

The probability that in a litter of six kittens, three will be black and three will be gray.

because gray is recessive. Thus the cross is:

1

Gg black /2 gg gray

/2 1

We can use the binomial expansion to determine the probability of obtaining three black and three gray kittens in a litter of six. Let p equal the probability of a kitten being black and q equal the probability of a kitten being gray. The binomial is (p + q)6, the expansion of which is:

Recall: The binomial expansion can be used to determine the probability of different combinations of traits in the progeny of a cross.

Hint: See pp. 56–57 for an explanation of how to expand the binomial.

( p + q)6 = p6 + 6p5q + 15p4q2 + 20p3q3 + 15p2q4 + 6p1q5 + q6 The probability of obtaining three black and three gray kittens in a litter of six is provided by the term 20p3q3. The probabilities of p and q are both 1/2 ; so the overall probability is 20(1/2) 3(1/2)3 = 20/64 = 5/16 .

Problem 3

In corn, purple kernels are dominant over yellow kernels, and full kernels are dominant over shrunken kernels. A corn plant having purple and full kernels is crossed with a plant having yellow and shrunken kernels, and the following progeny are obtained:

purple, full purple, shrunken yellow, full yellow, shrunken

112 103 91 94

What are the most likely genotypes of the parents and progeny? Test your genetic hypothesis with a chi-square test. Solution Strategy What information is required in your answer to the problem?

The genotypes of parents and progeny. A chi-square test comparing the observed and expected results. What information is provided to help solve the problem? ■

urple kernels are dominant over yellow kernels and P full kernels are dominant over shrunken kernels.

Pierce_5e_c03_045-076hr1.indd 69

The phenotypes of the parents. ■ The phenotypes and numbers of the different progeny of the cross. ■

For help with this problem, review:

Sections 3.3 and 3.4.

6/4/13 3:59 PM

70

Chapter 3

Solution Steps

The best way to begin this problem is by breaking the cross down into simple crosses for a single characteristic (seed color or seed shape): P purple Ž yellow

full Ž shrunken

F1 112 + 103 = 215 purple 112 + 91 = 203 full 91 + 94 = 185 yellow 103 + 94 = 197 shrunken Hint: A good strategy in a cross involving multiple characteristics is to analyze the results for each characteristic separately.

In this cross, purple Ž yellow produces approximately 1/2 purple and 1/2 yellow (a 1 : 1 ratio). A 1 : 1 ratio is usually caused by a cross between a heterozygote and a homozygote. Because purple is dominant, the purple parent must be heterozygous (Pp) and the yellow parent must be homozygous (pp). The purple progeny produced by this cross will be heterozygous (Pp) and the yellow progeny must be homozygous (pp). Now let’s examine the other character. Full Ž Recall: The multiplication shrunken produces 1/2 full and 1/2 shrunken, or a rule states that the probability of two or 1 : 1 ratio, so these progeny phenotypes are also more independent produced by a cross between a heterozygote (Ff ) and events occurring together is calculated a homozygote ( f f ); the full-kernel progeny will be by multiplying their inheterozygous (Ff ) and the shrunken-kernel progeny dependent probabilities. will be homozygous ( f f ). Now combine the two crosses and use the multiplication rule to obtain the overall genotypes and the proportions of each genotype: P

Purple, full Pp Ff

Yellow, shrunken pp ff

Ž

F1 Pp Ff = 1/2 purple

Ž

1

Pp ff = 1/2 purple

Ž

1

Ž

1

Ž

1

1

pp Ff = /2 yellow pp ff = 1/2 yellow

/2 full = 1/4 purple, full /2 shrunken = 1/4 purple, shrunken /2 full = 1/4 yellow, full /2 shrunken = 1/4 yellow, shrunken

Our genetic explanation predicts that, from this cross, we should see 1/4 purple, full-kernel progeny; 1/4 purple, shrunken-kernel progeny; 1/4 yellow, full-kernel progeny; and 1/4 yellow, shrunken-kernel progeny. A total of 400 progeny were produced; so 1/4 Ž 400 = 100 of each phenotype are expected. These observed numbers do not fit the expected numbers exactly. Could the difference between what we observe and what we expect be due to chance? If the probability is high that chance alone is responsible for the difference between observed and expected, we will assume that the progeny have been

Pierce_5e_c03_045-076hr.indd 70

produced in the 1 : 1 : 1 : 1 ratio predicted by the cross. If the probability that the difference between observed and expected is due to chance is low, the progeny are not really in the predicted ratio and some other, significant factor must be responsible for the deviation. The observed and expected numbers are: Phenotype Observed Expected 1 purple, full 112 /4 Ž 400 = 100 1 purple, shrunken 103 /4 Ž 400 = 100 1 yellow, full 91 /4 Ž 400 = 100 1 yellow, shrunken 94 /4 Ž 400 = 100 To determine the probability that the difference between observed and expected is due to chance, we calculate a chi-square value with the formula 2 = [(observed - expected)2/expected]: 2 =

1 112 - 100 2 2

+

1 103 - 100 2 2

+

1 91 - 100 2 2

100 100 122 32 92 62 = + + + 100 100 100 100 144 9 81 36 = + + 100 100 100 100 = 1.44 + 0.09 + 0.81 + 0.36 = 2.70

100

+

Hint: See Figure 3.13 for help on how to carry out a chi-square test.

1 94 - 100 2 2 100

Now that we have the chi-square value, we must determine the probability that this chi-square value is due to chance. To obtain this probability, we first calculate the degrees of freedom, which for a chi-square goodness-of-fit test are n - 1, where n equals the number of expected phenotypic classes. In this case, there are four expected phenotypic classes; so the degrees of freedom equal 4 - 1 = 3. We must now look up the chi-square value in a chi-square table (see Table 3.7). We select the row corresponding to 3 degrees of freedom and look along this row to find our calculated chi-square value. The calculated chi-square value of 2.7 lies between 2.366 (a probability of 0.5) and 6.251 (a probability of 0.1). The probability (P) associated with the calculated chi-square value is therefore 0.5 , P , 0.1. This is the probability that the difference between what we observed and what we expect is due to chance, which in this case is relatively high, and so chance is likely responsible for the deviation. We can conclude that the progeny do appear in the 1 : 1 : 1 : 1 ratio predicted by our genetic explanation.

5/3/13 3:12 PM

Basic Principles of Heredity

71

COMPrehension QUESTIONS Section 3.1

Section 3.3

1. Why was Mendel’s approach to the study of heredity so successful? 2. What is the difference between genotype and phenotype?

9. What is the principle of independent assortment? How is it related to the principle of segregation? 10. In which phases of mitosis and meiosis are the principles of segregation and independent assortment at work?

Section 3.2

3. What is the principle of segregation? Why is it important? 4. How are Mendel’s principles different from the concept of blending inheritance discussed in Chapter 1? 5. What is the concept of dominance? 6. What are the addition and multiplication rules of probability and when should they be used? 7. Give the genotypic ratios that may appear among the progeny of simple crosses and the genotypes of the parents that may give rise to each ratio. 8. What is the chromosome theory of heredity? Why was it important?

Section 3.4

11. How is the chi-square goodness-of-fit test used to analyze genetic crosses? What does the probability associated with a chi-square value indicate about the results of a cross?

For more questions that test your comprehension of the key for this chapter. chapter concepts, go to

APPLICATION QUESTIONS AND PROBLEMS Introduction

12. The inheritance of red hair was discussed in the introduction to this chapter. At times in the past, red hair in humans was thought to be a recessive trait and, at other times, it was thought to be a dominant trait. What features of heritance would red hair be expected to exhibit as a recessive trait? What features would it be expected to exhibit if it were a dominant trait? Section 3.1

* 13. What characteristics of an organism would make it suitable for studies of the principles of inheritance? Can you name several organisms that have these characteristics?

Cross tan Ž red

F1 F2 13 tan seeds 93 tan, 24 red seeds

a. Explain the inheritance of tan and red seeds in this plant. b. Assign symbols for the alleles in this cross and give genotypes for all the individual plants. * 17. White (w) coat color in guinea pigs is recessive to black DATA (W). In 1909, W. E. Castle and J. C. Phillips transplanted ANALYSIS an ovary from a black guinea pig into a white female whose ovaries had been removed. They then mated this white female with a white male. All the offspring from the mating were black in color (W. E. Castle and J. C. Phillips. 1909. Science 30:312–313).

Section 3.2

14. In cucumbers, orange fruit color (R) is dominant over cream fruit color (r). A cucumber plant homozygous for orange fruit is crossed with a plant homozygous for cream fruit. The F1 are intercrossed to produce the F2 . a. Give the genotypes and phenotypes of the parents, the F1, and the F2 . b. Give the genotypes and phenotypes of the offspring of a backcross between the F1 and the orange-fruited parent. c. Give the genotypes and phenotypes of a backcross between the F1 and the cream-fruited parent. 15. Figure 1.1 (p. 2) shows three girls, one of whom has albinism. Could the three girls shown in the photograph be sisters? Why or why not? 16. J. W. McKay crossed a stock melon plant that produced DATA tan seeds with a plant that produced red seeds and obtained the following results (J. W. McKay. 1936. ANALYSIS Journal of Heredity 27:110–112).

Pierce_5e_c03_045-076hr3.indd 71

[Wegner/ARCO/Nature Picture Library; Nigel Cattlin/Alamy.]

a. Explain the results of this cross. b. Give the genotype of the offspring of this cross. c. What, if anything, does this experiment indicate about the validity of the pangenesis and the germ-plasm theories discussed in Chapter 1? * 18. In cats, blood-type A results from an allele (IA) that is dominant over an allele (iB) that produces blood-type B. There is no O blood type. The blood types of male and female cats that were mated and the blood types of their

6/18/13 3:10 PM

72

Chapter 3

kittens follow. Give the most likely genotypes for the parents of each litter. a. b. c. d. e. f.

Male Female parent parent Kittens A B 4 with type A, 3 with type B B B 6 with type B B A 8 with type A A A 7 with type A, 2 with type B A A 10 with type A A B 4 with type A, 1 with type B

19. Figure 3.7 shows the results of a cross between a tall pea plant and a short pea plant. a. What phenotypes and proportions will be produced if a tall F1 progeny is backcrossed to the short parent? b. What phenotypes and proportions will be produced if a tall F1 progeny is backcrossed to the tall parent?

[Jeffrey van daele/FeaturePics.]

20. Joe has a white cat named Sam. When Joe crosses Sam with a black cat, he obtains 1/2 white kittens and 1/2 black kittens. When the black kittens are interbred, all the kittens that they produce are black. On the basis of these results, would you conclude that white or black coat color in cats is a recessive trait? Explain your reasoning. * 21. In sheep, lustrous fleece results from an allele (L) that is dominant over an allele (l) for normal fleece. A ewe (adult female) with lustrous fleece is mated with a ram (adult male) with normal fleece. The ewe then gives birth to a single lamb with normal fleece. From this single offspring, is it possible to determine the genotypes of the two parents? If so, what are their genotypes? If not, why not? * 22. Alkaptonuria is a metabolic disorder in which affected persons produce black urine. Alkaptonuria results from an allele (a) that is recessive to the allele for normal metabolism (A). Sally has normal metabolism, but her brother has alkaptonuria. Sally’s father has alkaptonuria, and her mother has normal metabolism. a. Give the genotypes of Sally, her mother, her father, and her brother. b. If Sally’s parents have another child, what is the probability that this child will have alkaptonuria? c. If Sally marries a man with alkaptonuria, what is the probability that their first child will have alkaptonuria? 23. Suppose that you are raising Mongolian gerbils. You notice that some of your gerbils have white spots, whereas others have solid coats. What type of crosses

Pierce_5e_c03_045-076hr.indd 72

could you carry out to determine whether white spots are due to a recessive or a dominant allele? 24. Hairlessness in American rat terriers is recessive to the presence of hair. Suppose that you have a rat terrier with hair. How can you determine whether this dog is homozygous or heterozygous for the hairy trait? * 25. What is the probability of rolling one six-sided die and obtaining the following numbers? a. 2 b. 1 or 2 c. An even number d. Any number but a 6 * 26. What is the probability of rolling two six-sided dice and obtaining the following numbers? a. 2 and 3 b. 6 and 6 c. At least one 6 d. Two of the same number (two 1s, or two 2s, or two 3s, etc.) e. An even number on both dice f. An even number on at least one die * 27. In a family of seven children, what is the probability of obtaining the following numbers of boys and girls? a. All boys b. All children of the same sex c. Six girls and one boy d. Four boys and three girls e. Four girls and three boys 28. Phenylketonuria (PKU) is a disease that results from a recessive gene. Two normal parents produce a child with PKU. a. What is the probability that a sperm from the father will contain the PKU allele? b. What is the probability that an egg from the mother will contain the PKU allele? c. What is the probability that their next child will have PKU? d. What is the probability that their next child will be heterozygous for the PKU gene? * 29. In German cockroaches, curved wing (cv) is recessive to normal wing (cv+). A homozygous cockroach having normal wings is crossed with a homozygous cockroach having curved wings. The F1 are intercrossed to produce the F2 . Assume that the pair of chromosomes containing the locus for wing shape is metacentric. Draw this pair of chromosomes as it would appear in the parents, the F1, and each class of F2 progeny at metaphase I of meiosis. Assume that no crossing over takes place. At each stage, label a location for the alleles for wing shape (cv and cv+) on the chromosomes.

5/3/13 3:12 PM

* 30. In guinea pigs, the allele for black fur (B) is dominant over the allele for brown (b) fur. A black guinea pig is crossed with a brown guinea pig, producing five F1 black guinea pigs and six F1 brown guinea pigs. a. How many copies of the black allele (B) will be present in each cell of an F1 black guinea pig at the following stages: G1, G2, metaphase of mitosis, metaphase I of meiosis, metaphase II of meiosis, and after the second cytokinesis following meiosis? Assume that no crossing over takes place. b. How many copies of the brown allele (b) will be present in each cell of an F1 brown guinea pig at the same stages as those listed in part a? Assume that no crossing over takes place. Section 3.3

31. In watermelons, bitter fruit (B) is dominant over sweet fruit (b), and yellow spots (S) are dominant over no spots (s). The genes for these two characteristics assort independently. A homozygous plant that has bitter fruit and yellow spots is crossed with a homozygous plant that has sweet fruit and no spots. The F1 are intercrossed to produce the F2 . a. What are the phenotypic ratios in the F2? b. If an F1 plant is backcrossed with the bitter, yellow-spotted parent, what phenotypes and proportions are expected in the offspring? c. If an F1 plant is backcrossed with the sweet, nonspotted parent, what phenotypes and proportions are expected in the offspring? 32. Figure 3.10 shows the results of a dihybrid cross involving seed shape and seed color. a. What proportion of the round and yellow F2 progeny from this cross is homozygous at both loci? b. What proportion of the round and yellow F2 progeny from this cross is homozygous at least at one locus? * 33. In cats, curled ears result from an allele (Cu) that is dominant over an allele (cu) for normal ears. Black color results from an independently assorting allele (G) that is dominant over an allele for gray (g). A gray cat homozygous for curled ears is mated with a homozygous black cat with normal ears. All [Biosphoto/J.-L. Klein & the F1 cats are black and have M.-L. Hubert/Peter Arnold.] curled ears. a. If two of the F1 cats mate, what phenotypes and proportions are expected in the F2? b. An F1 cat mates with a stray cat that is gray and possesses normal ears. What phenotypes and proportions of progeny are expected from this cross?

Pierce_5e_c03_045-076hr.indd 73

Basic Principles of Heredity

73

* 34. The following two genotypes are crossed: Aa Bb Cc dd Ee Ž Aa bb Cc Dd Ee. What will the proportion of the following genotypes be among the progeny of this cross? a. Aa Bb Cc Dd Ee b. Aa bb Cc dd ee c. aa bb cc dd ee d. AA BB CC DD EE 35. In mice, an allele for apricot eyes (a) is recessive to an allele for brown eyes (a+). At an independently assorting locus, an allele for tan coat color (t) is recessive to an allele for black coat color (t+). A mouse that is homozygous for brown eyes and black coat color is crossed with a mouse having apricot eyes and a tan coat. The resulting F1 are intercrossed to produce the F2 . In a litter of eight F2 mice, what is the probability that two will have apricot eyes and tan coats? 36. In cucumbers, dull fruit (D) is dominant over glossy fruit (d), orange fruit (R) is dominant over cream fruit (r), and bitter cotyledons (B) are dominant over nonbitter cotyledons (b). The three characters are encoded by genes located on different pairs of chromosomes. A plant homozygous for dull, orange fruit and bitter cotyledons is crossed with a plant that has glossy, cream fruit and nonbitter cotyledons. The F1 are intercrossed to produce the F2 . a. Give the phenotypes and their expected proportions in the F2 . b. An F1 plant is crossed with a plant that has glossy, cream fruit and nonbitter cotyledons. Give the phenotypes and expected proportions among the progeny of this cross. * 37. Alleles A and a are located on a pair of metacentric chromosomes. Alleles B and b are located on a pair of acrocentric chromosomes. A cross is made between individuals having the following genotypes: Aa Bb Ž aa bb. a. Draw the chromosomes as they would appear in each type of gamete produced by the individuals of this cross. b. For each type of progeny resulting from this cross, draw the chromosomes as they would appear in a cell at G1, G2, and metaphase of mitosis. Section 3.4

* 38. J. A. Moore investigated the inheritance of spotting DATA patterns in leopard frogs (J. A. Moore. 1943. Journal of ANALYSIS Heredity 34:3–7). The pipiens phenotype had the normal spots that give leopard frogs their name. In contrast, the burnsi phenotype lacked spots on its back. Moore carried out the following crosses, producing the progeny indicated Parent phenotypes burnsi Ž burnsi burnsi Ž pipiens burnsi Ž pipiens

Progeny phenotypes 39 burnsi, 6 pipiens 23 burnsi, 33 pipiens 196 burnsi, 210 pipiens

5/3/13 3:12 PM

74

Chapter 3

a. On the basis of these results, what is the most likely mode of inheritance of the burnsi phenotype? b. Give the most likely genotypes of the parent in each cross (use B for the burnsi allele and B+ for pipiens allele) c. Use a chi-square test to evaluate the fit of the observed numbers of progeny to the number expected on the basis of your proposed genotypes. 39. In the 1800s, a man with dwarfism who lived in Utah DATA produced a large number of descendants: 22 children, ANALYSIS 49 grandchildren, and 250 great-grandchildren (see the illustration of a family pedigree to the right), many of whom also were dwarfs (F. F. Stephens. 1943. Journal of Heredity 34:229–235). The type of dwarfism found in this family is called Schmid-type metaphyseal chondrodysplasia, although it was originally thought to be achondroplastic dwarfism. Among the families of this kindred, dwarfism appeared only in members who had one parent with dwarfism. When one parent was a dwarf, the following numbers of children were produced. Family in which one parent had dwarfism A B C D E F G H I J K L M N O Total

Children with normal stature 15 4 1 6 2 8 4 2 0 3 2 2 2 1 0 52

Children with dwarfism 7 6 6 2 2 4 4 1 1 1 3 1 0 0 2 40

a. With the assumption that Schmid-type metaphyseal chondrodysplasia is rare, is this type of dwarfism inherited as a dominant or recessive trait? Explain your reasoning? b. On the basis of your answer for part a, what is the expected ratio of dwarf and normal children in the families given in the table. Use a chi-square test to determine if the total number of children for these families (52 normal, 40 dwarfs) is significantly different from the number expected.

Pierce_5e_c03_045-076hr.indd 74

Men Women Dwarfism

I II III

IV

IV

V V

VI VII VI

[Adapted from The Journal of Heredity 34:232.]

c. Use chi-square tests to determine if the number of children in family C (1 normal, 6 dwarf) and the number in family D (6 normal and 2 dwarf) are significantly different from the numbers expected on the basis of your proposed type of inheritance. How would you explain these deviations from the overall ratio expected? 40. Pink-eye and albino are two recessive traits found DATA in the deer mouse Peromyscus maniculatus. In mice ANALYSIS with pink-eye, the eye is devoid of color and appears pink from the blood vessels within it. Albino mice are completely lacking color both in their fur and in their eyes. F. H. Clark crossed pink-eyed mice with albino mice; the resulting F1 had normal coloration in their fur and eyes. He then crossed these F1 mice with mice that were pink eyed and albino and obtained the following mice. It is very hard to distinguish between mice that are albino and mice that are both pink-eye and albino, and so he combined these two phenotypes (F. H. Clark. 1936. Journal of Heredity 27:259–260). Number Phenotype of progeny wild-type fur, wild-type eye color 12 wild-type fur, pink-eye 62 albino 78 f albino, pink-eye Total 152 a. Give the expected numbers of progeny with each phenotype if the genes for pink-eye and albino assort independently.

5/3/13 3:12 PM

Basic Principles of Heredity

b. Use a chi-square test to determine if the observed numbers of progeny fit the number expected with independent assortment. * 41. In the California poppy, an allele for yellow flowers (C) is dominant over an allele for white flowers (c). At an independently assorting locus, an allele for entire petals (F) is dominant over an allele for fringed petals ( f ). A plant that is homozygous for yellow and entire petals is crossed with a plant that is white and fringed. A resulting F1 plant is then crossed with a plant that is white and

75

fringed, and the following progeny are produced: 54 yellow and entire; 58 yellow and fringed, 53 white and entire, and 10 white and fringed. a. Use a chi-square test to compare the observed numbers with those expected for the cross. b. What conclusion can you make from the results of the chi-square test? c. Suggest an explanation for the results.

CHALLENGE QUESTIONS

42. Dwarfism is a recessive trait in Hereford cattle. A rancher in western Texas discovers that several of the calves in his herd are dwarfs, and he wants to eliminate this undesirable trait from the herd as rapidly as possible. Suppose that the rancher hires you as a genetic consultant to advise him on how to breed the dwarfism trait out of the herd. What crosses would you advise the rancher to conduct to ensure that the allele causing dwarfism is eliminated from the herd? * 43. A geneticist discovers an obese mouse in his laboratory colony. He breeds this obese mouse with a normal mouse. All the F1 mice from this cross are normal in size. When he interbreeds two F1 mice, eight of the F2 mice are normal in size and two are obese. The geneticist then intercrosses two of his obese mice, and he finds that all of the progeny from this cross are obese. These results lead the geneticist to conclude that obesity in mice results from a recessive allele. A second geneticist at a different university also discovers an obese mouse in her laboratory colony. She carries out the same crosses as those done by the first geneticist and obtains the same results. She also concludes that obesity in mice results from a recessive allele. One day the two geneticists meet at a genetics conference, learn of each other’s experiments, and decide to exchange mice. They both find that, when they cross two obese mice from the different laboratories, all the offspring are normal; however, when they cross two obese mice from the same laboratory, all the offspring are obese. Explain their results. 44. Albinism is a recessive trait in humans (see the introduction to Chapter 1). A geneticist studies a series of families in which both parents are normal and at least one child has albinism. The geneticist reasons that both parents in these families must be heterozygotes and that albinism should appear in 1/4 of the children of these families. To his surprise, the geneticist finds that the frequency of albinism among

Pierce_5e_c03_045-076hr5.indd 75

the children of these families is considerably greater than 1/4 . Can you think of an explanation for the higher-than-expected frequency of albinism among these families? 45. Two distinct phenotypes are found in the salamander Plethodon cinereus: a red form and a black form. Some biologists have speculated that the red phenotype is due to an autosomal allele that is dominant over an allele for black. Unfortunately, these salamanders will not mate in captivity; so the hypothesis that red is dominant over black has never been tested. One day a genetics student is hiking through the forest and finds 30 female salamanders, some red and some black, laying eggs. The student places each female and her eggs (from about 20 to 30 eggs per female) in separate plastic bags and takes them back to the lab. There, the student successfully raises the eggs until they hatch. After the eggs have [George Grall/National Geographic/ hatched, the student Getty Images.] records the phenotypes of the juvenile salamanders, along with the phenotypes of their mothers. Thus, the student has the phenotypes for 30 females and their progeny, but no information is available about the phenotypes of the fathers. Explain how the student can determine whether red is dominant over black with this information on the phenotypes of the females and their offspring.

▼

Section 3.2

Go to your

to find additional learning

resources and the Suggested Readings for this chapter.

7/23/13 10:22 AM

Pierce_5e_c03_045-076hr.indd 76

5/3/13 3:12 PM

Our partners will collect data and use cookies for ad personalization and measurement. Learn how we and our ad partner Google, collect and use data. Agree & close