Epistasis is a Non-Mendelian phenomenon where the expression of one gene/trait is dependent on the expression of another gene/trait i.e. the gene will only be expressed in the presence/absence of another one.
- It is non-mendelian because from Mendel's second law; The Law of Independent Assortment, genes are supposed to be inherited independently. This is an example of gene interaction.
- The gene that is being masked is the hypostatic gene while the gene influencing the expression of the hypostatic gene is known as the epistatic gene
Epistasis can be;
- Dominant
- Recessive
Dominant Epistasis
This is when the epistatic allele is a dominant allele.
- In this case, expression of the epistatic gene in homozygous dominant or heterozygous form is enough to inhibit the expression of the hypostatic gene.
- Expression of the epistatic gene in homozygous recessive form has no effect and in this case, the hypostatic gene will be expressed.
I will give an example below:
Summer squash is a fruit that has three colours; white, yellow and green. The yellow trait(Y) is dominant while green(y) is recessive. The 'W' gene codes for the white colour and is dominantly epistatic to the 'Y' gene. This means once the dominant W allele is present, no colour will be expressed.
W___ codes for white squash, wwY_ codes for