Epistasis

Epistasis = Interaction of different gene loci.

One gene locus masks or surpresses the expression of another gene locus. 

The genes involved may control the expression of 1 phenotypic characteristic by either:

• Working AGAINST each other (ANTAGONISTICALLY) resulting in masking.
• Working TOGETHER in a complementary fashion.

Working antagonistically:

• Homozygous presence of a recessive allele may prevent expression of another allele at a second locus. 
• Alleles at 1st locus are epistatic to alleles at the 2nd locus (hypostatic). 

Epistasis reduces phenotypic variation. 

Recessive:

• E.g. a homozygous aa may be epistatic to alleles of the gene B/b. 
• Neither the B or the b can be expressed if there is no dominant allele A present.

Dominant:

• Occurs when a dominant allele at one gene locus masks the expression of the alleles at a 2nd gene locus. 
• E.g. two gene oci D/d and E/e. Presence of one D alleleresults in whire fruits regardless of the aleles present at the 2nd locus. 

Working in a complementary fashion:

• The way 2 gene loci may produce these results is if they compliment each other. 
• If one gene codes for an intermediate colourless pigment and the second locus codes for an enzyme that converts the intermediate compound to the final purple pigment.