16. A black Labrador homozygous for the dominant alleles (BBEE) is crossed with a yellow Labrador homozygous for the recessive alleles (bbee). On intercrossing the F1, the F2 progeny was obtained in the following ratio: 9 black: 3 brown: 4 yellow. This is an example of
(1) recessive epistasis where allele e is epistatic to B and b.
(2) dominant epistasis where allele E is epistatic to B and b.
(3) recessive epistasis where allele e is epistatic to E.
(4) complementary epistasis where allele b is epistatic

The correct answer is:
(1) recessive epistasis where allele e is epistatic to B and b.

In Labrador coat colour genetics, two loci interact:

• B/b locus: controls type of dark pigment.

  • B_ = black

  • bb = brown (chocolate)

• E/e locus: controls deposition of dark pigment in hair.

  • E_ = allows expression of B/b (black or brown)

  • ee = blocks deposition of dark pigment, giving yellow coat regardless of B or b.

From the cross BBEE (black) × bbee (yellow), F1 are all BbEe (black). Intercrossing F1 (BbEe × BbEe) gives classic recessive epistasis 9:3:4:

• 9 B_E_ = black

• 3 bbE_ = brown

• 4 __ee (B_ee + bbee) = yellow.

Here, the homozygous recessive ee genotype masks (is epistatic to) both B and b alleles, so this is recessive epistasis by e over B/b.

Now, option-by-option:

1. recessive epistasis where allele e is epistatic to B and b

• Matches the biology: ee (recessive) prevents expression of black or brown pigment, making the dog yellow regardless of B or b.

• Produces the observed 9 black : 3 brown : 4 yellow ratio, so this option is correct.

2. dominant epistasis where allele E is epistatic to B and b

• Would mean any E_ genotype masks B/b; that is not true in Labradors.

• E_ actually permits, not blocks, expression of B/b, giving black (B_E_) or brown (bbE_).

• This would not yield 9:3:4, so the statement is incorrect.

3. recessive epistasis where allele e is epistatic to E

• E and e are alleles at the same locus, so one cannot be epistatic “to E”; epistasis is between different loci.

• The interaction is between the E locus and the B locus, not within E/e itself, so this wording is wrong.

4. complementary epistasis where allele b is epistatic

• Complementary (9:7) epistasis requires both dominant alleles from two loci for a trait; that is not the Labrador pattern.

• Allele b does not mask B; bb simply converts black to brown when E_ is present, and its effect is itself masked by ee.

• Therefore this description does not fit the 9:3:4 ratio or the biology.

Introduction:

In Labrador Retrievers, three familiar coat colours—black, brown (chocolate), and yellow—are controlled by interaction between two genes, B/b and E/e. When a BBEE black Labrador is crossed with a bbee yellow Labrador and the F1 (BbEe) dogs are intercrossed, the F2 puppies appear in a 9 black : 3 brown : 4 yellow ratio, a textbook example of recessive epistasis at the E locus.

How the B and E genes control coat colour:

The B gene determines the type of dark pigment: dogs with at least one B allele (B_) are black, whereas bb dogs are brown, provided pigment is allowed in the coat. The E gene regulates whether this dark pigment appears in the fur; E_ allows pigment deposition, while ee blocks it, resulting in a yellow coat even though the skin pigment still reflects the B or b genotype. Thus, genotype combinations group as B_E_ (black), bbE_ (brown), and __ee (yellow).

Deriving the 9:3:4 ratio in the F2:

Intercrossing F1 BbEe × BbEe produces 16 genotype combinations following independent assortment. When grouped by phenotype, 9 genotypes contain at least one B and one E allele (B_E_) and are black, 3 genotypes are bbE_ and therefore brown, and 4 genotypes are ee (B_ee or bbee) and appear yellow because ee masks the B/b locus. This classification produces the observed 9 black : 3 brown : 4 yellow ratio, confirming recessive epistasis of ee over B and b.

Why this is recessive epistasis by e:

Recessive epistasis occurs when a homozygous recessive genotype at one locus hides the effect of another locus, and that is precisely what happens with ee in Labradors. Whenever ee is present, the phenotype is yellow, and the underlying B or bb genotype cannot be seen in the coat, though it is still present genetically; only when at least one E allele is present does the B locus determine whether the dog is black or brown. Therefore, this pattern is correctly described as “recessive epistasis where allele e is epistatic to B and b.”