EPISTASIS

=Epistasis= So far we have looked at cases where two genes control two distinct characteristics. However, most characteristics are controlled by more than one gene. Usually, there is an underlying biochemical pathway which is mediated by several proteins or enzymes which, in turn, are each coded for by a single gene. [image:http://i.imgur.com/uo6bnTB.png] The above enzymes are both coded for by different genes found on completely different chromosomes. For simplicity sake we can assume '''enzyme A''' is coded for by '''gene A''' and that the recessive form of the gene (the '''a''' allele) does not code for a functional protein at all. Similarly '''enzyme B''' is coded for by the '''gene B''' and that the recessive form of the gene (the '''b''' allele) also does not code for a functional protein. ''To produce chemical compound 2 the individual will need at least one dominant allele for gene A'' ''To produce chemical compound 3 the individual will need at least one dominant allele for both gene A & B'' Therefore the presence of one gene can mask the effect of the other. For instance if an individual has the genotype ''aaBB'' none of the third chemical compound is produced. The absence of the '''A''' allele masks the effect of the '''B''' allele.