Epistasis: Gene Interaction and Phenotype

Epistasis is the interaction between different genes, not between alleles of the same gene, and it affects the phenotypic ratios. It occurs when one gene masks or affects the phenotype of another.

A modifier gene, also known as an epistatic gene, affects the expression of another gene, either by inactivating its product, counteracting its effects, or removing it from the structure needed for the phenotype.

In recessive epistasis, a homozygous recessive allele at one locus prevents the other locus from exerting its phenotype, whereas in dominant epistasis, a dominant allele at one locus prevents the other locus from exhibiting the homozygous recessive phenotype.

In duplicate recessive epistasis, the 9:3:3:1 ratio is modified to 9:7, as either locus being homozygous recessive masks the dominant effect at the other locus.

In duplicate dominant recessive epistasis, the presence of a dominant allele at either of two loci produces the dominant phenotype, and the products from either A or B gene perform the same function.

In duplicate gene epistasis with cumulative effect, the 9:3:3:1 ratio is modified to 9:6:1 as the dominant allele at either locus, but not both, produces the same phenotype.

6 different types

1. recessive epistasis
2. dominant epistasis
3. dominant and recessive epistasis
4. duplicate recessive epistasis
5. duplicate dominant recessive epistasis
6. duplicate gene epistasis with cumulative effect.

9:3:4

e.g. flower colour in snap dragons C gene controls the production of pigment P gene controls the deposition of pigment C is epistatic to P gene meaning presence of cc masks the effect of P.

12:3:1 E.g. coat colour in some animals A controls production of pigment in fur B controls the distribution of pigment in fur B is epistatic to A meaning presence of dominant allele B masks the effect of allele A

13:3