Department of Animal Science - Basic Animal Genetics

Epistasis occurs when genes at two different loci interact to affect the expression of a single trait. A gene can either mask or modify the phenotype controlled by the other gene. As you read this text, you can click on the animation to the right to observe all of the genotypes and phenotypes that we discuss.

Masking epistasis occurs when a gene at one locus masks the expression of a gene at the second locus so its phenotype is not expressed. Modifying epistasis occurs when a gene at one locus modifies or changes the expression of the phenotype of a gene at the second locus. The gene that does the masking/modifying is referred to as epistatic, while the gene that is masked/modified is referred to as hypostatic.

Recessive masking epistasis can be observed in the coat color of Labrador Retrievers. There are three possible genotypes that result in different coat colors:

• B_E_ is a black Lab
• bbE_ is a brown Lab
• _ _ ee is a yellow Lab

Notice that if the animal inherits the homozygous recessive genotype of the "E" gene, "ee", then it will be yellow regardless of whether it is dominant or recessive for the "B" gene. So the recessive genotype of "ee" masks the phenotypic expression of the "B" gene.

Dominant masking epistasis can be observed in horse coat color. There are four possible genotypes that result in different coat colors:

• G_ _ _ is a gray horse at maturity
• gg_ _ is a horse that is not gray
• gg E _ is a black horse
• ggee is a chestnut horse

Notice that if the genotype is dominant for the "G" gene, then the horse will be gray at maturity because the "G" gene is masking the phenotypic expression of the gene at the second locus. If the "G" genotype is homozygous recessive ("gg"), then the phenotype of the gene at the second locus can be expressed and the horse will not be gray. In the example the "E" genotype, depending on whether it is dominant or recessive, results in a black or chestnut horse.

Modifying epistasis can be observed in the coat color of Dobermans. There are four possible genotypes that result in four different coat colors.

• B_D_ is a black dog
• bbD_ is a red dog
• B_dd is a blue (faded black) dog
• bbdd is a fawn (faded red) dog

Notice that if the "D" genotype is recessive ("dd"), it acts to modify the phenotypic expression of the "B" gene. The "dd" genotype modifies the "B" gene by fading its color, therefore creating new coat colors.