A form of gene interaction in which one gene affects the expression of a second gene. It usually arises because the two genes affect sequential steps in the same biochemical pathway. It is most easily detected as a modification 01 the Mendelian ratio in the F 2. The phenomenon was first reported in the inheritance of flower colour. For example, suppose there are two enzymes, A' and B', produced by the normal alleles of two unlinked genes, A and B, but not by their recessive alleles, a and b. The substrate of A' is a white pigment and the product of B' is a purple pigment. In addition the product of A', a red pigment, is also the substrate of B'. Thus the sequence of reactions white pigment to red pigment to purple pigment can only take place if there is at least one normal allele at both gene loci. The genotype aa__would give a white phenotype (no enzyme A'; gene B is immaterial), genotype A_bb would give a red phenotype (enzyme A' but not B'), and genotype A_B_ would give a purple phenotype (both A' and B'). Instead of the F 2 forming a 9:3:3:1 ratio, a 9:3:4 ratio (= 9:3:[3 + l]) would be obtained, since aaB_ would be indistinguishable from aabb. In this case, gene A is the epistatic gene, and gene B the hypostatic gene. Other variants are 9:7, 13:3, and 12:3:1 ratios. See also complementary genes. |
