Q2Biology
Question
4 marks
Define Epistasis. Explain recessive epistasis with the help of suitable example giving the Ratio of Phenotype and Genotype.
Answer
Epistasis is the masking of one gene's effect by another non-allelic gene; in recessive epistasis, the homozygous recessive genotype at one locus suppresses the expression of alleles at a second locus, giving a 9:3:4 ratio.
- Cross: BbEe x BbEe (dihybrid)
- F2 phenotypic ratio: 9 B_E_ (black) : 3 bbE_ (brown) : 3 B_ee (yellow) : 1 bbee (yellow) = 9:3:4
- Genotypic ratio remains 1:2:1:2:4:2:1:2:1 (standard 9 genotypic classes)
- The 3:1 for black:yellow in the recessive class collapses to give 9:3:4 overall.