BIOLOGY Volume 3 - A Guide to General Biology - 2004

24. VARIATION AND GENETICS

24.7. Gene Interactions

24.7.5. Epistasis

A Gene is called epistatic (from the Greek epi — upon, over, and stasis — standing, stoppage) if its presence masks or suppresses The Effect of a gene at a different locus. Epistatic genes are sometimes referred to as inhibiting genes because they exert an effect on other genes, which are known as hypostatic (from the Greek hypó — under and stasis).

Coat color in mice is controlled by a pair of genes located at different loci. The epistatic gene determines whether color is produced at all and has two alleles: a dominant allele that allows pigmentation, and a recessive allele that results in albinism (white coat). The hypostatic gene determines the type of pigment and has two alleles: agouti (dominant, producing gray banding) and black (recessive). Mice can have either agouti or black coats depending on their genotypes, but the simultaneous presence of the pigmentation-allowing allele is required for any color to be expressed. Mice homozygous for the recessive albinism allele will be albinos regardless of whether they carry the agouti or black alleles. Three distinct phenotypes are possible: agouti, black, and albino. Depending on the genotypes of the parents, crossbreeding can yield these phenotypes in various ratios (Figs. 24.28 and Table 24.7).

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Fig. 24.27. Genetic explanation of comb shape inheritance in chickens.

Fig. 24.28. Genetic explanation of unusual coat color phenotype ratios resulting from epistasis in mice.

Table 24.7. Some phenotypic ratios observed in the offspring resulting from epistatic interactions of coat color genes in mice (see also Fig. 24.28)

24.15. In White Leghorn chickens, plumage color is controlled by two gene pairs:

W (white plumage) is dominant to w (colored)

B (black plumage) is dominant to b (brown)

The heterozygous F₁ offspring have the genotype WwBb and are white.

Explain the gene interactions occurring in this case and determine the phenotypic ratio in the F₂ generation.