ANALYTICAL APPROACHES FOR SOLVING GENETICS PROBLEMS

02.1 A purple-flowered pea plant is crossed with a white-flowered pea plant. All the F₁ plants produced purple flowers. When the F₁ plants are allowed to self-pollinate, 401 of the F₂s have purple flowers and 131 have white flowers. What are the genotypes of the parental and F₁ generation plants?

02.2 Consider three gene pairs Aa, Bb, and Cc, each of which affects a different character. In each case the uppercase letter signifies the dominant allele and the lowercase letter the recessive allele. These three gene pairs assort independently of each other. Calculate the probability of obtaining:

a. an Aa BB Cc zygote from a cross of individuals that are Aa Bb Cc x Aa BbCc;

b. an Aa BB cc zygote from a cross of individuals that are aa BBcc x AA bbCC;

c. an A B C phenotype from a cross of individuals that are Aa Bb CC x Aa Bb cc;

d. an a b c phenotype from a cross of individuals that are Aa Bb Cc x aa Bb cc.

Q2.3 In chickens the white plumage of the leghorn breed is dominant over colored plumage, feathered shanks are dominant over clean shanks, and pea comb is dominant over single comb. Each of the gene pairs segregates independently. If a homozygous white, feathered, pea-combed chicken is crossed with a homozygous colored, clean, single-comb chicken, and the F₁s are allowed to interbreed, what proportion of the birds in the F₂ will produce only white, feathered, pea-combed progeny if mated to colored, clean-shanked, single-combed birds?

2.4 F₂ plants segregate 3/4 colored:1/4 colorless. If a colored plant is picked at random and selfed, what is the probability that more than one type will segregate among a large number of its progeny?

2.5 In guinea pigs rough coat (R) is dominant over smooth coat (r). A rough-coated guinea pig is bred to a smooth one, giving eight rough and seven smooth progeny in the F₁.

a. What are the genotypes of the parents and their offspring?

b. If one of the rough F₁ animals is mated to its rough parent, what progeny would you expect?

2.6 In cattle the polled (hornless) condition (P) is dominant over the horned (p) phenotype. A particular polled bull is bred to three cows. Cow A, which is homed, produces a horned calf; a polled cow B produces a homed calf; and horned cow C produces a polled calf. What are the genotypes of the bull and the three cows, and what phenotypic ratios do you expect in the offspring of these three matings?

2.13 In Jimsonweed, purple flower (P) is dominant to white (p), and spiny pods (S) are dominant to smooth (s). In a cross between a Jimsonweed homozygous for white flowers and spiny pods and one homozygous for purple flowers and smooth pods, determine the phenotype of (a) the F₁ (b) the F₂ (c) the progeny of a cross of the F₁ back to the white, spiny parent; (d) the progeny of a cross of the F₁ back to the purple, smooth parent.