Week 2: Chapter 3: Homework

24: From a presumed testcross A/a × a/a, in which A repre- sents red and a represents white, use the χ2 test to find out which of the following possible results would fit the expectations: a. 120 red, 100 white b. 5000 red, 5400 white c. 500 red, 540 white d. 50 red, 54 white

-- a, b & d would fit the expectations -- c would not

38: A dark female moth is crossed with a dark male. All the male progeny are dark, but half the female progeny are light and the rest are dark. Propose an explanation for this pattern of inheritance.

In moths, the heterogametic sex is actually the female, while the male is homogametic. -- the dark male is heterozygous (D/d) -- the dark female is hemizygous (D) -- all male progeny will inherit the D allele from their mother and therefore be dark -- while half the females will inherit D from their father (and be dark) and half will inherit d (and be light)

20: In mice, dwarfism is caused by an X-linked recessive allele, and pink coat is caused by an autosomal dominant allele (coats are normally brownish). If a dwarf female from a pure line is crossed with a pink male from a pure line, what will be the phenotypic ratios in the F1 and F2 in each sex? (Invent and define your own gene symbols.)

Keep in mind that the question asks for phenotypic ratios. Use terms such as wild type.

37: Are the following progeny numbers consistent with the results expected from selfing a plant presumed to be a dihybrid of two independently assorting genes, H/h ; R/r? (H = hairy leaves; h = smooth leaves; R = round ovary; r = elongated ovary.) Explain your answer. hairy, round 178 smooth, round 56 hairy, elongated 62 smooth, elongated 24

The hypothesis is that the organism being tested is a dihybrid with independently assorting genes and that all progeny are of equal viability. The expected values would be that phenotypes occur in a 9:3:3:1 ratio. There are four phenotypes so there are 3 degrees of freedom.