Some genes are involved in regulating the expression of other genes or groups of genes.
Mendel's two-factor crosses worked well because the pairs of genes involved did not interact with each other.In many instances the 9:3:3:1 ratio Mendel observed is not found when other loci or organisms such as bacteria are studied in similar crosses.
Sometimes pairs of alleles control different steps in producing the same trait. These are known as epistatic genes or alleles. Epistasis refers to one gene's interfering with the expression of another.
An example of epistatic genes at work is seen in mouse hair color. One pair of alleles produces brown (bb) or black (BB or Bb), but another pair permits (CC, Cc) or prevents (cc) the presence of color. When the color preventer (a recessive) is homozygous, the mice are white (BBcc or bbcc for example).
In the epistatic cross BbCc x BbCc, the phenotype ratio in the offspring turns out to be 9:3:4 instead of the classic 9:3:3:1 because every offspring with a cc genotype is white (9 black: 3 brown: 4 white.
This page was modified Dec. 26, 2001