GENETICS (BIO 325) ________________________________________________________________ Chap 6 (Gene Interaction) 1. Dominant epistasis (12: 3: 1) When the dominant allele at one locus, for example the A allele, produces a certain phenotype regardless of the allelic condition of the other locus, then the A-locus is said to be epistatic to the B-locus. Furthermore, since the dominant allele A is able to express itself in the presence of either B or b, this is a case of dominant epistasis. Only when the genotype of the individual is homozygous recessive at the epistatic locus (a/a) can the alleles of the hypostatic locus (B or b) be expressed. Thus the genotypes A/-; B/- and A/-; b/b produce the same phenotype, whereas a/a: B/- and a/a; b/b produce two additional phenotypes. The classical 9: 3: 3: 1 ratio becomes modified into a 12: 3: 1 ratio. 2. Recessive epistasis (9: 3: 4) If the recessive genotype at one locus (for example, a/a) suppresses the expression of alleles at the B-locus, the A-locus is said to exhibit recessive
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