Page 93 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

Problems 85

f. Now consider independent pathways as in (a), but the situation depicted in Fig. 3.28b shows only five
the presence of compound 2 masks the colors due possible phenotypic classes. How can you explain this
to all other compounds. difference in the amount of phenotypic variation?
g. Next consider the sequential pathway shown in (c), 40. Three genes in fruit flies affect a particular trait, and
but compounds 1 and 2 are the same color. one dominant allele of each gene is necessary to get a
h. Finally, examine the pathway that follows. Here, wild-type phenotype.
compounds 1 and 2 have different colors. The pro- a. What phenotypic ratios would you predict among
tein encoded by A prevents the conversion of com- the progeny if you crossed triply heterozygous flies?
pound 1 to compound 2. The protein encoded by B b. You cross a particular wild-type male in succession
prevents protein A from functioning. with three tester strains. In the cross with one tester
strain (AA bb cc), only 1/4 of the progeny are wild
Protein B Protein A
type. In the crosses involving the other two tester
strains (aa BB cc and aa bb CC), half of the prog-
eny are wild type. What is the genotype of the
Compound 1 Compound 2 wild-type male?
37. Considering your answers to Problem 36, does the ex- 41. The garden flower Salpiglossis sinuata (painted
istence of a particular variation of a 9:3:3:1 ratio tongue) comes in many different colors. Several
among the F 2 progeny allow you to infer the operation crosses are made between true-breeding parental
of a specific biochemical mechanism responsible for strains to produce F 1 plants, which are in turn self-
these phenotypes? Inversely, if you know a biochemi- fertilized to produce F 2 progeny.
cal mechanism of gene interaction, can you predict
the ratios of the phenotypes you would see among the Parents F 1 phenotypes F 2 phenotypes
F 2 progeny? red × blue all red 102 red, 33 blue
lavender × blue all lavender 149 lavender, 51 blue
Section 3.3 lavender × red all bronze 84 bronze, 43 red, 41 lavender
38. You picked up two mice (one female and one male) red × yellow all red 133 red, 58 yellow, 43 blue
that had escaped from experimental cages in the animal yellow × blue all lavender 183 lavender, 81 yellow, 59 blue
facility. One mouse is yellow in color, and the other is a. State a hypothesis explaining the inheritance of
brown agouti. (Agouti hairs have bands of yellow, flower color in painted tongues.
while non-agouti hairs are solid-colored.) You know b. Assign genotypes to the parents, F 1 progeny, and
that this mouse colony has animals with different al- F 2 progeny for all five crosses.
leles at only three coat color genes: the agouti (A) or
non-agouti (a) or yellow (A ) alleles of the A gene c. In a cross between true-breeding yellow and true-
Y
(A > A > a; A is a recessive lethal), the black (B) or breeding lavender plants, all of the F 1 progeny are
Y
Y
brown (b) alleles of the B gene (B > b), and the albino bronze. If you used these F 1 plants to produce an
(c) or non-albino (C) alleles of the C gene (C > c; cc is F 2 generation, what phenotypes in what ratios
epistatic to all other phenotypes). However, you don’t would you expect? Are there any genotypes that
know which alleles of these genes are actually present might produce a phenotype that you cannot predict
in each of the animals that you’ve captured. To deter- from earlier experiments, and if so, how might this
mine the genotypes, you breed the two escaped mice to- alter the phenotypic ratios among the F 2 progeny?
gether. The first litter has only three pups. One is albino, 42. In foxgloves, three different petal phenotypes exist:
one is brown (non-agouti), and the third is black agouti. white with red spots (WR), dark red (DR), and light
a. What alleles of the A, B, and C genes are present in red (LR). Two different kinds of true-breeding WR
the two mice you caught? strains (WR-1 and WR-2) can be distinguished by
b. After raising several litters from these two parents, two-generation intercrosses with true-breeding DR
and LR strains:
you have many offspring. How many different coat
color phenotypes (in total) do you expect to see ex- F 2
pressed in the population of offspring? What are Parental F 1 WR LR DR
the phenotypes and corresponding genotypes? 1 WR-1 × LR all WR 480 39 119
39. Figure 3.21 and Fig. 3.28b both show traits that are 2 WR-1 × DR all WR 99 0 32
determined by two genes, each of which has two in- 3 DR × LR all DR 0 43 132
completely dominant alleles. But in Fig. 3.21 the gene 4 WR-2 × LR all WR 193 64 0
interaction produces nine different phenotypes, while 5 WR-2 × DR all WR 286 24 74