Interactions between genes

One of the features of Mendelian genetics is that each gene has a distinct phenotype which is unaffected by any other gene. A gene specifies whether the flower will be purple or white. Another gene specifies if the seed will be green or yellow. A third specifies if the plant is tall or short.

The phenotype of each allele of each gene is unaffected by what alleles are present at other genes. This independence is not always true, sometimes the genes interact--interactions between distinct genes is called epistasis.

An example of epistasis:

A gene in labradors controls deposition of melanin. The dominant allele B causes deposition of large amounts of melanin. A recessive allele b causes less deposition. So A BB or Bb dog is black, while a bb dog is brown

Another gene controls whether or not melanin is deposited at all.

The dominant allele E allows deposition. The recessive allele e does not
so An ee dog is yellow (no melanin) while an Ee or EE dog has melanin and is not yellow.

Combining any combinations of alleles of the B gene with ee results in a yellow dog rather than a black or brown dog. This is not dominance since gene E and B are not allelic (dominance is a relationship between homologous genes). However, homozygous recessive mutants of E can not express any melanin, and therefore the nature of the B genes is irrelevant to coat color. This kind of relationship is called epistasis.

In a dihybrid cross between BB EE and bb ee you get:

9 Black (combination of at least one B and one E allele)
3 Brown (homozygous bb combined with at least one E allele)
4 Yellow (homozygous ee with any alleles of B).

Commonly, a gene is entirely epistatic to another, as with the B and E genes. The example of comb shape in chickens is an example of an epistatic interaction which is similar to "co-dominance".

A gene called Rose (R) allows growth of a complex, wide comb rather than a simple flat comb. A gene called Pea (P) controls the size of the comb, keeping it small.

A homozygous recessive mutation rr gives the a flat comb which is either small (called a pea comb) when combined with at least one P gene, or very large (single comb) when combined with homozygous pp.

A homozygous recessive mutation pp gives a large comb, which is either complex (rose comb) when combined with at least one P gene, or flat when combined with homozygous rr (single comb). The combination of dominant alleles gives a small, complex comb (called walnut). The epistasis relationship between the two genes gives the four comb types: walnut, rose, pea and single.

More complicated interactions lead to continuous variation in phenotypes:

Eye color and height are two examples. In each case, the phenotype can not be broken down into two clear alternatives. Instead multiple genes interact to affect the phenotype. Another name for this type of trait is multigenic