Chapter 10

A certain recessive allele \(c\) is responsible for albinism, an inability to produce or deposit melanin, a brownishblack pigment, in body tissues. Humans and a number of other organisms can have this phenotype. Figure 10.18 shows two stunning examples. In cases of albinism, what are the possible genotypes of the father, the mother, and their children? a. Both parents have normal phenotypes; some of their children are albino and others are unaffected. b. Both parents are albino and have albino children. c. The woman is unaffected, the man is albino, and they have one albino child and three unaffected children.

One gene has alleles \(A\) and \(a\). Another has alleles \(B\) and \(b\). For each genotype, what type(s) of gametes will form? Assume that independent assortment occurs. a. \(A A B B\) b. \(A a B B\) c. \(A a b b\) d. \(A a B b\)

Certain dominant alleles are so essential for normal development that an individual who is homozygous recessive for a mutant recessive form can't survive. Such recessive, lethal alleles can be perpetuated in the population by heterozygotes. Consider the Manx allele \(\left(M^{L}\right)\) in cats. Homozygous cats \(\left(M^{L} M^{L}\right)\) die when they are still embryos inside the mother cat. In heterozygotes \(\left(M^{L} M\right),\) the spine develops abnormally. The cats end up with no tail (Figure 10.19 ). Two \(M^{L} M\) cats mate. What is the probability that any one of their surviving kittens will be heterozygous?

In one experiment, Mendel crossed a pea plant that bred true for green pods with one that bred true for yellow pods. All the \(F_{1}\) plants had green pods. Which form of the trait (green or yellow pods) is recessive? Explain how you arrived at your conclusion.

Return to Problem 2. Assume you now study a third gene having alleles \(C\) and \(c .\) For each genotype listed, what type(s) of gametes will be produced? a. \(A A B B C C\) c. \(A a B B C c\) b. \(A a B B c c\) d. \(A a B b C c\)

Mendel crossed a true-breeding tall, purple-flowered pea plant with a true- breeding dwarf, white-flowered plant. All \(\mathrm{F}_{1}\) plants were tall and had purple flowers. If an \(\mathrm{F}_{1}\) plant self-fertilizes, then what is the probability that a randomly selected \(\mathrm{F}_{2}\) offspring will be heterozygous for the genes specifying height and flower color?

DNA fingerprinting is a method of identifying individuals by locating unique base sequences in their DNA molecules (Section 15.4 ). Before researchers refined the method, attorneys often relied on the ABO bloodtyping system to settle disputes over paternity. Suppose that you, as a geneticist, are asked to testify during a paternity case in which the mother has type A blood, the child has type \(\mathrm{O}\) blood, and the alleged father has type \(\mathrm{B}\) blood. How would you respond to the following statements? a. Attorney of the alleged father: "The mother's blood is type \(A,\) so the child's type \(O\) blood must have come from the father. My client has type B blood; he could not be the father." b. Mother's attorney: "Because further tests prove this man is heterozygous, he must be the father."

Suppose you identify a new gene in mice. One of its alleles specifies white fur. A second allele specifies brown fur. You want to determine whether the relationship between the two alleles is one of simple dominance or incomplete dominance. What sorts of genetic crosses would give you the answer? On what types of observations would you base your conclusions?

Your sister gives you a purebred Labrador retriever, a female named Dandelion. Suppose you decide to breed Dandelion and sell puppies to help pay for your college tuition. Then you discover that two of her four brothers and sisters show hip dysplasia, a heritable disorder arising from a number of gene interactions. If Dandelion mates with a male Labrador known to be free of the harmful alleles, can you guarantee to a buyer that her puppies will not develop the disorder? Explain your answer.

A dominant allele \(W\) confers black fur on guinea pigs. A guinea pig that is homozygous recessive \((ww)\) has white fur. Fred would like to know whether his pet black-furred guinea pig is homozygous dominant \((WW)\) or heterozygous \((W w) .\) How might he determine his pet's genotype?

12\. Red-flowering snapdragons are homozygous for allele \(R^{1}\). White-flowering snapdragons are homozygous for a different allele \(\left(R^{2}\right)\). Heterozygous plants \(\left(R^{1} R^{2}\right)\) bear pink flowers. What phenotypes should appear among first-generation offspring of the crosses listed? What are the expected proportions for each phenotype? a. \(R^{1} R^{1} \times R^{1} R^{2}\) b. \(R^{1} R^{1} \times R^{2} R^{2}\) c. \(R^{1} R^{2} \times R^{1} R^{2}\) d. \(R^{1} R^{2} \times R^{2} R^{2}\) (In cases of incomplete dominance, alleles are usually designated by superscript numerals, as shown here, not by the uppercase letters for dominance and lowercase letters for recessiveness.

Two pairs of genes affect comb type in chickens (Figure 10.12 ). When both genes are recessive, a chicken has a single comb. A dominant allele of one gene, \(P,\) gives rise to a pea comb. Yet a dominant allele of the other \((R)\) gives rise to a rose comb. An epistatic interaction occurs when a chicken has at least one of both dominants, \(P_{-} R_{-}\) which gives rise to a walnut comb. Predict the ratios resulting from a cross between two walnut-combed chickens that are heterozygous for both genes \((P p R r)\)

As Section 3.6 explains, a single mutant allele gives rise to an abnormal form of hemoglobin (Hb \(S\) instead of \(\left.H b^{A}\right) .\) Homozygotes \((H b S H b S)\) develop sickle-cell anemia. Heterozygotes \(\left(H b^{A} H b S\right)\) show few obvious symptoms. Suppose a woman's mother is homozygous for the \(H b^{A}\) allele. She marries a male who is heterozygous for the allele, and they plan to have children. For each of her pregnancies, state the probability that this couple will have a child who is: a. homozygous for the \(H b^{S}\) allele b. homozygous for the \(H b^{A}\) allele c. heterozygous \(H b^{A} H b^{S}\)