Page 70 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

62 Chapter 3 Extensions to Mendel’s Laws

Figure 3.18 Dominant epistasis may also result in a 13:3 phenotypic ratio. (a) In the F 2 generation resulting from a dihybrid
cross between white leghorn and white wyandotte chickens, the ratio of white birds to birds with color is 13:3. This ratio emerges because
at least one copy of A and the absence of B is needed to produce color. (b) Enzyme A, encoded by allele A, is needed to synthesize
pigment. Allele a encodes no enzyme. Pigment deposition in the feathers depends on protein b encoded by allele b, the normal (wild-type)
allele of a second gene. The mutant dominant allele B, however, encodes an abnormal version of the protein that prevents pigment
deposition, even when the normal protein b is present.
(a) B is epistatic to A (b) Biochemical explanation for dominant epistasis in the
White White generation of chicken feather color
leghorn wyandotte BB, Bb
P AA BB aa bb AA, Aa
Protein B
Colorless Enzyme A No pigment
Gametes A B a b precursor Pigment deposited

aa
F (all identical) Aa Bb Aa Bb BB, Bb
1
No enzyme A
Protein B
Colorless No No pigment
F 2 precursor pigment deposited
A B A b a B a b
aa
A B AA BB AA Bb Aa BB Aa Bb
(9) A– B– bb
13 (white) No enzyme A
(3) aa B–
A b AA Bb AA bb Aa Bb Aa bb Protein b
(1) aa bb Colorless No No pigment
3 A– bb (colored) precursor pigment deposited
a B Aa BB Aa Bb aa BB aa Bb
bb
AA, Aa
a b Aa Bb Aa bb aa Bb aa bb
Protein b
Colorless Enzyme A Pigment
precursor Pigment deposited

The squash genes A and B have not been identified at pathway known to underly the 13:3 ratio for chicken
the molecular level, and the biochemical pathway in which feather color is shown in Fig. 3.18b.
they interact is unknown. However, based on knowledge of
similar phenomena in other plants, a likely biochemical Important points regarding epistasis
pathway underlying the 12:3:1 phenotypic ratio is shown in
Fig. 3.17b. Several important points emerge from the examples of
recessive and dominant epistasis we have discussed:
Chicken feather color A variant ratio indicating domi- ∙ Epistasis is an interaction between alleles of different
nant epistasis is seen in the feather color of certain chick- genes, not between alleles of the same gene.
ens (Fig. 3.18a). White leghorns have a doubly dominant ∙ In dihybrid crosses, the F 2 phenotypic ratios result-
AA BB genotype for feather color; white wyandottes are ing from epistasis depend on the functions of the
homozygous recessive for both genes (aa bb). A cross be- specific alleles and the particular biochemical path-
tween these two pure-breeding white strains produces an ways in which the genes participate.
all-white dihybrid (Aa Bb) F 1 generation, but birds with
color in their feathers appear in the F 2 , and the ratio of In the Labrador retriever and sweet pea examples of
white to colorful is 13:3 (Fig. 3.18a). We can explain this recessive epistasis, the completely dominant alleles of both
ratio by assuming a kind of dominant epistasis in which B genes specify normally functional protein, while the reces-
is epistatic to A; the A allele produces color only in the sive alleles are either nonfunctional or specify weakly
absence of B; and the a, B, and b alleles produce no color. functional protein. Nevertheless, the phenotypic ratios
The interaction is characterized by a 13:3 ratio because the among the F 2 of a dihybrid cross differ in the Labradors
9 A– B–, 3 aa B–, and 1 aa bb genotypic classes combine and peas because the underlying biochemical pathways
to produce only one phenotype: white. The biochemical are not identical. Likewise, the two dominant epistasis