Coat color in Labradors involves two genes. The B locus controls pigment (B black, b brown). The E locus controls deposition (E pigment deposited, ee no deposition resulting in yellow regardless of B).
The 9:3:4 ratio indicates which type of gene interaction?
- A
Codominance between B and E.
- B
Independent assortment producing a 9:3:3:1 ratio.
- C
Incomplete dominance at the E locus.
- Dcheck_circle
Recessive epistasis: ee masks expression at the B locus.
Explanation
Homozygous ee blocks pigment deposition regardless of B genotype, masking the B-locus phenotype. The combined yellow class (3 BbEe-yellow + 1 bbee-yellow = 4) yields the modified 9:3:4 ratio characteristic of recessive epistasis.