Question
Explain the concept of epistasis using the example of coat colour in Labrador Retrievers. Include the genotypes for each coat colour and the modified F₂ ratio observed.
Solution — Step by Step
Epistasis is a form of gene interaction where one gene masks or suppresses the expression of another gene at a different locus. It is different from dominance, which involves alleles at the same locus. In epistasis, we are dealing with two separate genes where one overrides the other.
The word comes from Greek: “epi” = upon, “stasis” = standing — one gene “stands upon” another and silences it.
Labrador coat colour is controlled by two independently assorting genes:
Gene 1 — Pigment production (B/b):
- B (dominant): Black pigment (eumelanin) produced
- b (recessive): Brown/chocolate pigment produced
Gene 2 — Pigment deposition (E/e):
- E (dominant): Pigment deposited in fur (colour is expressed)
- e (recessive, homozygous ee): Pigment NOT deposited — dog is yellow regardless of B/b genotype
So gene E is the epistatic gene — the homozygous recessive (ee) suppresses/masks the effect of gene B. This is called recessive epistasis.
| Genotype | Coat Colour | Explanation |
|---|---|---|
| B_E_ | Black | Has B (black pigment) + E (deposits pigment) |
| bbE_ | Chocolate/Brown | Has bb (brown pigment) + E (deposits pigment) |
| B_ee | Yellow | Has B (black pigment) but ee blocks deposition |
| bbee | Yellow | Has bb (brown pigment) but ee blocks deposition |
Both B_ee and bbee appear yellow because the ee genotype prevents any pigment from reaching the fur, regardless of which pigment gene is present.
Start with a cross between Black (BBEE) × Yellow (bbee):
F₁: All BbEe (Black — because B is dominant over b, E is dominant over e)
F₁ × F₁ cross (BbEe × BbEe):
Using independent assortment (9:3:3:1 from two genes):
- 9 B_E_ = Black
- 3 bbE_ = Chocolate
- 3 B_ee = Yellow (epistasis — B masked by ee)
- 1 bbee = Yellow (epistasis — bb masked by ee)
The last two classes (3 + 1) both appear yellow because of the ee condition.
Modified ratio: 9 Black : 3 Chocolate : 4 Yellow
This 9:3:4 is the characteristic ratio for recessive epistasis.
Epistasis is called recessive here because it is the homozygous recessive genotype (ee) that causes the masking. When at least one E allele is present (EE or Ee), the dog can show its B/b colour. Only when both E alleles are absent (ee) does the masking occur.
Compare this with dominant epistasis (12:3:1 ratio) where a single dominant allele at one locus masks the other gene’s expression.
Why This Works
The key insight: a gene can only produce an observable effect if the entire biochemical pathway leading to that phenotype is functional. In Labrador colour:
- Gene B controls which pigment is made (black vs brown)
- Gene E controls whether that pigment gets deposited in the fur
If the “deposition” step (E gene) is broken (ee), it doesn’t matter what colour the pigment is — no pigment reaches the fur. The dog looks yellow (the default/diluted colour from small amounts of pheomelanin in the fur).
This is biochemical epistasis — one gene controls an upstream step and another controls a downstream step in the same pathway.
Alternative Method
You can draw a 4×4 Punnett square for BbEe × BbEe to get all 16 combinations, then classify each by phenotype. Count: 9 have B_E_, 3 have bbE_, 3 have B_ee, 1 has bbee. Group the ee genotypes together → 9:3:4.
Common Mistake
Students often confuse epistasis with dominance. Dominance is between alleles of the same gene (B dominant over b). Epistasis is between different genes (E gene masks B gene). When writing about Labrador coat colour in NEET, explicitly state “Gene E is epistatic to Gene B” — don’t use the word dominance here.