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What are the different types of epistasis?

Epistasis occurrs when one allele of a gene masks the expression of alleles of another gene.

When there is no epistasis a dihybrid cross (two characteristics) of two heterozygote individuals(each individual has one of each allele) results in a phenotypic ratio or 9:3:3:1 (both dominant:first dominant, second recessive:second dominant, first recessive:both recessive).

here are the different types:

1. Recessive epistasis, i.e. the epistatic allele is recessive so for it to mask the other gene two copies are needed. To illustrate this carry out a dihybrid cross with a homozygous dominant individual and a homozygous recessive individual and you will see a ratio of 9:3:4 (dominant both: dominant epistatic, recessive other:recessive epistatic).

 

2. Dominant epistasis, i.e. the epistatic allele is dominant so only one copy is needed to mask the other gene. If you carry out the same cross as for recessive you will see a ratio of 12:3:1 (dominant epistatic: recessive epistatic, dominant other: recessive both).

 

3. Complementary epistasis, i.e. the genes work together in a complementary fashion so you need at least one dominant allele of both genes to get one phenotype and all other combinations give another phenotype. The ratio you get is 9:7 (dominant both: recessive either or both). 

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