1. Using the Punnett squares below, show that for typical dominant and recessive autosomal traits, it does not matter which parent contributes the dominant allele and which the recessive allele. Cross true-breeding tall plants (TT) with true-breeding dwarf plants (it).
Male Female gametes gametes
Male Female gametes gametes
2. The photograph shows the shells of 15 bay scallops, Argopecten irradians. These scallops are hermaphroditic; that is, a single individual can reproduce sexually, as did the pea plants of the F1 generation in Mendel's experiments. Three color schemes are evident: yellow, orange, and black and white. The color-determining gene has three alleles. The top row shows a yellow scallop and a representative sample of its offspring, the middle row shows a black-and-white scallop and its offspring, and the bottom row shows an orange scallop and its offspring. Assign a suitable symbol to each of the three alleles participating in color control; then determine the genotype of each of the three parent individuals and tell what you can about the genotypes of the different offspring. Explain your results carefully.
3. Show diagrammatically what occurs when the F1 offspring of the cross in Question 1 self-pollinate.
4. A new student of genetics suspects that a particular recessive trait in fruit flies (dumpy wings, which are somewhat smaller and more bell-shaped than the wild-type) is sex-linked. A sin-
gle mating between a fly having dumpy wings (dp; female) and a fly with wild-type wings (Dp; male) produces 3 dumpy-winged females and 2 wild-type males. On the basis of these data, is the trait sex-linked or autosomal? What were the genotypes of the parents? Explain how these conclusions can be reached on the basis of so few data.
5. The sex of fishes is determined by the same X-Y system as in humans. An allele of one locus on the Y chromosome of the fish Lebistes causes a pigmented spot to appear on the dorsal fin. A male fish that has a spotted dorsal fin is mated with a female fish that has an unspotted fin. Describe the pheno-types of the F1 and the F2 generations from this cross.
6. In Drosophila melanogaster, the recessive allele p, when homozygous, determines pink eyes. Pp or PP results in wildtype eye color. Another gene, on another chromosome, has a recessive allele, sw, that produces short wings when homozygous. Consider a cross between females of genotype PPSwSw and males of genotype ppswsw. Describe the phenotypes and genotypes of the F1 generation and of the F2 generation produced by allowing the F1 progeny to mate with one another.
7. On the same chromosome of Drosophila melanogaster that carries the p (pink eyes) locus, there is another locus that affects the wings. Homozygous recessives, byby, have blistery wings, while the dominant allele By produces wild-type wings. The P and By loci are very close together on the chromosome; that is, the two loci are tightly linked. In answering these questions, assume that no crossing over occurs.
a. For the cross PPByBy x ppbyby, give the phenotypes and genotypes of the F1 and of the F2 generations produced by interbreeding of the F1 progeny.
b. For the cross PPbyby x ppByBy, give the phenotypes and genotypes of the F1 and of the F2 generations.
c. For the cross of Question 7b, what further phenotype(s) would appear in the F2 generation if crossing over occurred?
d. Draw a nucleus undergoing meiosis, at the stage in which the crossing over (Question 7c) occurred. In which generation (P, F1, or F2) did this crossing over take place?
8. Consider the following cross of Drosophila melanogaster (alleles as described in Question 6): Males with genotype Ppswsw are crossed with females of genotype ppSwsw. Describe the phe-notypes and genotypes of the F1 generation.
9. In the Andalusian fowl, a single pair of alleles controls the color of the feathers. Three colors are observed: blue, black, and splashed white. Crosses among these three types yield the following results:
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