Sometimes the effect of gene interaction is that one gene masks (hides) the effect of another gene at a different locus, a phenomenon known as epistasis. This phenomenon is similar to dominance, except that dominance entails the masking of genes at the same locus (allelic genes). In epista-sis, the gene that does the masking is called the epistatic gene; the gene whose effect is masked is a hypostatic gene. Epistatic genes may be recessive or dominant in their effects.
Recessive epistasis Recessive epistasis is seen in the genes that determine coat color in Labrador retrievers. These dogs may be black, brown, or yellow; their different coat colors are determined by interactions between genes at two loci (although a number of other loci also help to determine coat color; see p. 000). One locus determines the type of pigment produced by the skin cells: a dominant allele B codes for black pigment, whereas a recessive allele b codes for brown pigment. Alleles at a second locus affect the deposition of the pigment in the shaft of the hair; allele E allows dark pigment (black or brown) to be deposited, whereas a recessive allele e prevents the deposition of dark pigment, causing the hair to be yellow. The presence of genotype ee at the second locus therefore masks the expression of the black and brown alleles at the first locus. The genotypes that determine coat color and their phenotypes are:
B_ E_ black bbE_ brown (frequently called chocolate)
B_ee yellow bbee yellow
If we cross a black Labrador homozygous for the dominant alleles with a yellow Labrador homozygous for the recessive alleles and then intercross the Fj, we obtain progeny in the F2 in a 9:3:4 ratio:
P BBEE X bbee black yellow
F2 9/16 B_E_ black /16 bbE_ brown 3/16 B-ee yell°wj 4/ yellow
1/16 bbee yellow 16
Notice that yellow dogs can carry alleles for either black or brown pigment, but these alleles are not expressed in their coat color.
In this example of gene interaction, allele e is epistatic to B and b, because e masks the expression of the alleles for black and brown pigments, and alleles B and b are hypostatic to e. In this case, e is a recessive epistatic allele, because two copies of e must be present to mask of the black and brown pigments.
Dominant epistasis Dominant epistasis is seen in the interaction of two loci that determine fruit color in summer squash, which is commonly found in one of three colors: yellow, white, or green. When a homozygous plant that produces white squash is crossed with a homozygous plant that produces green squash and the F1 plants are crossed with each other, the following results are obtained:
P plants with plants with white squash X green squash
Fi plants with white squash
Intercross j2/j6 plants with white squash 3/16plants with yellow squash j/j6plants with green squash
How can gene interaction explain these results?
In the F2, 12/16 or 3/4 of the plants produce white squash and 3/16 + 1/16 = 4/16 = 1/4 of the plants produce squash having color. This outcome is the familiar 3: 1 ratio produced by a cross between two heterozygous individuals, which suggests that a dominant allele at one locus inhibits the production of pigment, resulting in white progeny. If we use the symbol W to represent the dominant allele that inhibits pigment production, then genotype W_ inhibits pigment production and produces white squash, whereas ww allows pigment and results in colored squash.
Among those ww F2 plants with pigmented fruit, we observe 3/16 yellow and 1/16 green (a 3:1 ratio). This outcome is because a second locus determines the type of pigment produced in the squash, with yellow (Y_) dominant over green (yy). This locus is expressed only in ww plants, which lack the dominant inhibitory allele W. We can assign the genotype wwY_ to plants that produce yellow squash and the genotype wwyy to plants that produce green squash. The genotypes and their associated phenotypes are:
W_Y_ W_yy wwY_ wwyy white squash white squash yellow squash green squash
Allele W is epistatic to Y and y— it suppresses the expression of these pigment-producing genes. Wis a dominant epistatic allele because, in contrast with e in Labrador retriever coat color, a single copy of the allele is sufficient to inhibit pigment production.
Summer squash provides us with a good opportunity for considering how epistasis often arises when genes affect a series of steps in a biochemical pathway. Yellow pigment in the squash is most likely produced in a two-step biochemical pathway ( FIGURE 5.8). A colorless (white) compound (designated A in Figure 5.8) is converted by enzyme I into green compound B, which is then converted into compound C by enzyme II. Compound C is the yellow pigment in the fruit.
Plants with the genotype ww produce enzyme I and may be green or yellow, depending on whether enzyme II is present. When allele Yis present at a second locus, enzyme II is produced and compound B is converted into compound C, producing a yellow fruit. When two copies of y,which does not encode a functional form of enzyme II, are present, squash remain green. The presence of W at the first locus inhibits the conversion of compound A into compound B; plants with genotype W_ do not make compound B and their fruit remains white, regardless of which alleles are present at the second locus.
Many cases of epistasis arise in this way. A gene (such as W) that has an effect on an early step in a biochemical pathway will be epistatic to genes (such as Y and y) that affect subsequent steps, because the effect of the enzyme in the later step depends on the product of the earlier reaction.
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