Question 3.12

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?

Question 3.13

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?

Question 3.14

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.

Question 3.15

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?

Question 3.16

J. W. McKay crossed a stock melon plant that produced tan seeds with a plant that produced red seeds and obtained the following results (J. W. McKay. 1936. Journal of Heredity 27:110–112).

• 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.

Question 3.17

White (w) coat color in guinea pigs is recessive to black (W). In 1909, W. E. Castle and J. C. Phillips transplanted 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).

[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?

Question 3.18

In cats, blood-type A results from an allele (I^A) that is dominant over an allele (i^B) 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 kittens follow. Give the most likely genotypes for the parents of each litter.

Question 3.19

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?

Question 3.20

Joe has a white cat named Sam. When Joe crosses Sam with a black cat, he obtains white kittens and 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.

Question 3.21

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?

[Jeffrey van daele/FeaturePics.]

Question 3.22

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?

Question 3.23

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 could you carry out to determine whether white spots are due to a recessive or a dominant allele?

Question 3.24

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?

Question 3.25

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

Question 3.26

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

Question 3.27

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

Question 3.28

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?

Question 3.29

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.

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Question 3.30

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.

Question 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?

Question 3.32

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?

Question 3.33

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 the F₁ cats are black and have curled ears.

[Biosphoto/J.-L. Klein & M.-L. Hubert/Peter Arnold.]

• 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?

Question 3.34

The following two genotypes are crossed: Aa Bb Cc dd Ee × 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

Question 3.35

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?

Question 3.36

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.

Question 3.37

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 × 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.

Question 3.38

J. A. Moore investigated the inheritance of spotting patterns in leopard frogs (J. A. Moore. 1943. Journal of 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

• 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.

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Question 3.39

In the 1800s, a man with dwarfism who lived in Utah produced a large number of descendants: 22 children, 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.

• 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.
• 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?

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

Question 3.40

Pink-eye and albino are two recessive traits found in the deer mouse Peromyscus maniculatus. In mice 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).

• a. Give the expected numbers of progeny with each phenotype if the genes for pink-eye and albino assort independently.
• b. Use a chi-square test to determine if the observed numbers of progeny fit the number expected with independent assortment.

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Question 3.41

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 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.