Page 74 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

66 Chapter 3 Extensions to Mendel’s Laws

Figure 3.22 Specific breeding tests can help decide between hypotheses. Either of two models could explain the results of a
cross tracking coat color in mice. (a) In one hypothesis, two genes interact with recessive epistasis to produce a 9:3:4 ratio. (b) In the other
hypothesis, a single gene with incomplete dominance between the alleles generates the observed results. One way to decide between these
models is to cross each of several albino F 2 mice with true-breeding brown mice. The two-gene model predicts several different outcomes
depending on the – – cc albino’s genotype at the B gene. The one-gene model predicts that all progeny of all the crosses will be black.
(a) Hypothesis 1 (two genes with recessive epistasis) (b) Hypothesis 2 (one gene with incomplete dominance)

2 2
1 1
P BB cc bb CC P B B B B
Gametes B c b C Gametes B 2 B 1

F (all identical) F (all identical)
1
1
2 1
2 1
Bb Cc Bb Cc B B B B
F 2 F 2
2 1
2 2
1 1
90 B– C– 30 bb C– 30 B– cc 10 bb cc 40 B B 80 B B 40 B B
9 : 3 : 3 : 1 1 : 2 : 1

If two-gene hypothesis is correct: If one-gene hypothesis is correct:

F albino True-breeding F albino True-breeding
2
2
brown brown
2 2
1 1
–– cc bb CC B B B B
b C b C b C B 2
2 1
B c Bb Cc or B c Bb Cc or b c bb Cc B 1 B B
b c bb Cc

The answer is that we can use other types of crosses to Locus Heterogeneity: Mutations in Any
verify or refute the hypotheses. For instance, if the one- One of Several Genes May Cause the
gene hypothesis were correct, a mating of F 2 albinos with
pure-breeding brown mice similar to those of the parental Same Phenotype
generation would produce all black heterozygotes [brown Close to 50 different genes have mutant alleles that can
(BB) × albino (bb) = all black (Bb)] (Fig. 3.22b). But if the cause deafness in humans. Many genes generate the devel-
two-gene hypothesis is correct, with recessive mutations at opmental pathway that brings about hearing, and a loss of
an albino gene (called C) epistatic to all expression from function in any part of the pathway, for instance, in one
the B gene, different matings of pure-breeding brown small bone of the middle ear, can result in deafness. In
(bb CC) with the F 2 albinos (– – cc) will give different other words, it takes a dominant wild-type allele at each of
results—all progeny are black; half are black and half these 50 genes to produce normal hearing. Thus, deafness
brown; all are brown—depending on the albino’s genotype is a heterogeneous trait: A mutation at any one of a
at the B gene (see Fig. 3.22a). In fact, when the experiment number of genes can give rise to the same phenotype. We
is actually performed, the diversity of results confirms the saw earlier (Fig. 3.15b) that whiteness of sweet pea flowers
two-gene hypothesis. is also a heterogeneous trait; AA bb and aa BB flowers,