28. In a cross between two black Labrador retrievers the phenotypic ratio of the offspring is 9 black puppies to 3
chocolate puppies to 4 yellow puppies; this is an example of
1. Partial recessiveness
2. Incomplete penetrance
3. Incomplete dominance
4. Epistasis

Labrador Retriever Coat Color and the Genetics of Epistasis

If you’ve ever noticed that Labrador Retrievers commonly come in black, chocolate, or yellow, you’re witnessing a fascinating genetic phenomenon at play. In a particular genetic cross between two black Labradors, the resulting 9 black: 3 chocolate: 4 yellow phenotypic ratio among the offspring is not your standard Mendelian inheritance. Instead, it’s a textbook case of epistasis.

Let’s break it down.

What Is Epistasis?

Epistasis occurs when the expression of one gene is dependent on or masked by another gene. In other words, one gene modifies or completely blocks the expression of another gene, even if that second gene has dominant alleles present.

Coat Color in Labrador Retrievers: Two Genes at Work

Labrador coat color is determined by two separate gene loci:

1. B gene – determines the color of pigment:

   • B (black) is dominant over b (chocolate).

2. E gene – controls whether pigment is deposited in the fur:

   • E (allows pigment deposition) is dominant over e (prevents pigment deposition).

How the 9:3:4 Ratio Arises

Here’s how it works in the offspring:

• Black Labradors (B_E_) – have at least one dominant B and one dominant E.

• Chocolate Labradors (bbE_) – two recessive b alleles but at least one dominant E.

• Yellow Labradors (__ee) – any combination of B or b, but homozygous recessive ee blocks pigment deposition, resulting in yellow.

This leads to:

• 9 black (B_E_)

• 3 chocolate (bbE_)

• 4 yellow (includes both B_ee and bbee)

Hence, the yellow coat (ee) is epistatic to the B/b gene, because it masks the expression of black or chocolate color.

Evaluating the Options:

1. Partial recessiveness

   • Incorrect. This refers to situations where heterozygotes show a phenotype between two homozygotes.

2. Incomplete penetrance

   • Incorrect. This term refers to cases where an individual with a particular genotype does not express the expected phenotype.

3. Incomplete dominance

   • Incorrect. This occurs when heterozygotes express a blend of both traits, not seen here.

4. Epistasis

   • ✅ Correct Answer – because the E gene’s effect masks the B gene’s expression, leading to yellow Labradors regardless of their B/b alleles.

✅ Correct Answer:

(4) Epistasis

Why This Matters

Understanding epistasis is crucial for:

• Animal breeding

• Genetic counseling

• Exploring complex trait inheritance in humans and other species

In Labrador retrievers, it’s a prime real-world example of how genetics is rarely as simple as a single dominant or recessive trait. This elegant interplay of genes gives rise to the diversity we see in the breed today.