Interaction of Genes

The phenotypic ratios obtained by Mendel in garden peas demonstrate that one gene controls one character; of the two alleles of a gene, one allele is completely dominant over the other. Due to this the heterozygote has a phenotype identical to the homozygous parent. Soon after Mendel’s work was rediscovered, instances came to light where a gene was not producing an individual effect. On the contrary, genes were interacting with each other to produce novel phenotypes which did not exhibit dominance relationships observed in Mendel’s experiments. In one of the first cases reported by Kölreuter, the heterozygote showed a phenotype intermediate between the parental phenotypes. This was termed incomplete dominance or intermediate inheritance. 

In codominance the heterozygote expresses both the parental phenotypes equally. Sometimes a gene masks the expression of another gene at a different locus. This is known as epistasis. On still other occasions a gene does not completely mask another gene as in epistasis, but, in some way modifies the effect of the second gene. Known as modifying genes, such genes either enhance or suppress the expression of a different gene. Interaction between genes enables some genes to act together to produce an effect that neither gene can produce separately. Such genes are said to be complementary. There are genes that copy other genes, so to say, to produce a similar effect. Thus independent genes that produce the same effect are given the name duplicate genes. Lastly, but most serious are genes causing death. They are known as lethal genes.

Reference:

Ahluwalia, Karvita B. 2009. Genetics. New Delhi: New Age International.