<P> In dominant epistasis, one gene locus may determine yellow or green pigment as in the previous example: AA and Aa are yellow, and aa are green . A second locus determines whether a pigment precursor is produced (dd) or not (DD or Dd). Here, in a DD or Dd plant, the flowers will be colorless irrespective of the genotype at the A locus, because of the epistatic effect of the dominant D allele . Thus, in a cross between two AaDd plants, 3 / 4 of the plants will be colorless, and the yellow and green phenotypes are expressed only in dd plants . This produces a characteristic 12: 3: 1 ratio of white: yellow: green plants . </P> <P> Supplementary epistasis occurs when two loci affect the same phenotype . For example, if pigment color is produced by CC or Cc but not cc, and by DD or Dd but not dd, then pigment is not produced in any genotypic combination with either cc or dd . That is, both loci must have at least one dominant allele to produce the phenotype . This produces a characteristic 9: 7 ratio of pigmented to unpigmented plants . Complementary epistasis in contrast produces an unpigmented plant if and only if the genotype is cc and dd, and the characteristic ratio is 15: 1 between pigmented and unpigmented plants . </P> <P> Classical genetics considered epistatic interactions between two genes at a time . It is now evident from molecular genetics that all gene loci are involved in complex interactions with many other genes (e.g., metabolic pathways may involve scores of genes), and that this creates epistatic interactions that are much more complex than the classic two - locus models . </P> <P> The frequency of the heterozygous state (which is the carrier state for a recessive trait) can be estimated using the Hardy - Weinberg formula: p 2 + 2 p q + q 2 = 1 (\ displaystyle p ^ (2) + 2pq + q ^ (2) = 1) </P>

Explain the effects of dominant genes over recessive genes