13.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 inheritance 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?

14.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?

15.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 (F₁ individuals crossed with F₁ individuals) to produce the F₂.

16.Figure 1.1 shows three girls, one of whom has albinism. Could the three girls shown in the photograph be sisters? Why or why not?

17. 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).

*18. 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 (W. E. Castle and J. C. Phillips. 1909. Science 30:312–313).

19.In cats, blood type A results from an allele I^A, which is dominant over an allele i^B that produces blood type B. (There is no O blood type, as there is in humans.) The blood types of male and female cats that were mated and the blood types of their kittens are presented in the following table. Give the most likely genotypes for the parents of each litter.

20.Figure 3.6 shows the results of a cross between a tall pea plant and a short pea plant.

21.Joe has a white cat named Sam. When Joe crosses Sam with a black cat, he obtains ½ white kittens and ½ black kittens. When the black kittens are interbred, all the kittens that they produce are black. On the basis of these results, which coat color (white or black) in cats would you conclude is a recessive trait? Explain your reasoning.

*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.

23.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?

*24.What is the probability of rolling one six-sided die and obtaining the following numbers?

*25.What is the probability of rolling two six-sided dice and obtaining the following numbers?

26.Phenylketonuria (PKU) is a disease that results from a recessive gene. Two normal parents produce a child with PKU.

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*27.In German cockroaches, a curved wing (cv) is recessive to a normal wing (cv⁺). A homozygous cockroach that has normal wings is crossed with a homozygous cockroach that has 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.

*28.In guinea pigs, the allele for black fur (B) is dominant over the allele for brown fur (b). A black guinea pig is crossed with a brown guinea pig, producing five F₁ black guinea pigs and six F₁ brown guinea pigs.

29.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.

30.Figure 3.8 shows the results of a dihybrid cross involving seed shape and seed color.

*31.In cats, curled ears result from an allele (Cu) that is dominant over an allele for normal ears (cu). 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.

*32.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?

33.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 non-bitter cotyledons (b). The three characteristics 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 non-bitter cotyledons. The F₁ are intercrossed to produce the F₂.

*34.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.

*35. 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 has the normal spots that give leopard frogs their name. In contrast, the burnsi phenotype lacks spots on its back. Moore carried out the following crosses, producing the progeny indicated.

*36.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.

37. Many studies have suggested a strong genetic predisposition to migraine headaches, but the mode of inheritance is not clear. L. Russo and colleagues examined migraine headaches in several families, two of which are shown in the following pedigree (L. Russo et al. 2005. American Journal of Human Genetics 76:327–333). What is the most likely mode of inheritance for migraine headaches in these families? Explain your reasoning.

38.For each of the following pedigrees, give the most likely mode of inheritance, assuming that the trait is rare. Carefully explain your reasoning.

39. Ectrodactyly is a rare condition in which the fingers are absent and the hand is split. This condition is usually inherited as an autosomal dominant trait. Ademar Freire-Maia reported the appearance of ectrodactyly in a family in São Paulo, Brazil, whose pedigree is shown here. Is this pedigree consistent with autosomal dominant inheritance? If not, what mode of inheritance is most likely? Explain your reasoning.