Page 69 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

3.2 Extensions to Mendel for Two-Gene Inheritance 61

sequential biochemical reactions to change a colorless pre- Dominant epistasis
cursor into a purple pigment, only the A– B– genotypic class, Epistasis can also be caused by a dominant allele. Depend-
which produces active forms of both required enzymes, can ing on the details of the biochemical pathway involved,
generate colored flowers. The other three genotypic classes dominant epistasis can result in either of two different phe-
(A– bb, aa B–, and aa bb) become grouped together with notypic ratios.
respect to phenotype because they do not specify functional
forms of one or the other requisite enzyme and thus give rise Squash fruit color In summer squash, two genes influence
to no color, which is the same as white. It is easy to see how the color of the fruit (Fig. 3.17a). With one gene, the domi-
the 7 part of the 9:7 ratio encompasses the 3:3:1 of the nant allele (A–) determines yellow, while homozygotes for
9:3:3:1 F 2 ratio. the recessive allele (aa) are green. A second gene’s dominant
The 9:7 ratio is the phenotypic signature of this allele (B–) produces white, while bb fruit may be either yel-
type of reciprocal recessive epistasis in which the low or green, depending on the genotype of the first gene. In
dominant alleles of two genes acting together (A– B–) the interaction between these two genes, the presence of B
produce color or some other trait, while the other three hides the effects of either A– or aa, producing white fruit,
genotypic classes (A– bb, aa B–, and aa bb) do not (see and B– is thus epistatic to any genotype of the A gene. The
Fig. 3.15b). Given that the phenotype associated with recessive b allele has no effect on fruit color determined by
either allele A or allele B is purple, then we can say that gene A. Epistasis in which the dominant allele of one gene
aa is epistatic to B, and bb is epistatic to A. If the sweet hides the effects of another gene is called dominant epista-
peas are either aa or bb, their flowers will be white re- sis. In a cross between white F 1 dihybrids (Aa Bb), the F 2
gardless of whether or not they have a dominant allele phenotypic ratio is 12 white : 3 yellow : 1 green (Fig. 3.17a).
of the other gene. The 12 includes two genotypic classes: 9 A– B– and 3 aa B–.

Figure 3.17 Dominant epistasis may result in a 12:3:1 phenotypic ratio. (a) In summer squash, the dominant B allele causes
white color and is sufficient to mask the effects of any combination of A and a alleles. As a result, yellow (A–) or green (aa) color is
expressed only in bb individuals. (b) The A allele encodes enzyme A, while the a allele specifies no enzyme. Therefore, yellow pigment is
present in A– squash and green pigment in aa squash. Deposition of either pigment depends on protein b encoded by allele b, the normal
(wild-type) allele of a second gene. However, the mutant dominant allele B encodes an abnormal version B of this protein that prevents
pigment deposition, even when the normal protein b is present. Therefore, in order to be colored, the squash must have protein b but not
protein B (genotype bb).
(a) B is epistatic to A and a. (b) A possible biochemical explanation for dominant epistasis in the
generation of summer squash color
BB, Bb
AA, Aa
P AA BB aa bb
Protein B
Green Enzyme A Yellow No pigment
Gametes A B a b pigment pigment deposited

aa
F 1 (all identical) BB, Bb
Aa Bb Aa Bb No enzyme A
Protein B
Green No yellow No pigment
pigment pigment deposited
F 2
A B A b a B a b
AA, Aa bb
A B AA BB AA Bb Aa BB Aa Bb
(9) A– B– Yellow
12 (white) Green Enzyme A Yellow Protein b
(3) aa B– pigment pigment pigment
A b AA Bb AA bb Aa Bb Aa bb deposited
3 A– bb (yellow)
1 aa bb (green)
a B Aa BB Aa Bb aa BB aa Bb aa
bb
No enzyme A
a b Aa Bb Aa bb aa Bb aa bb Green
Green No yellow Protein b pigment
pigment pigment
deposited