Question

In dogs, the allele for black coat color (B) is dominant to the allele for brown coat color (b). But if a dog has two copies of the recessive allele for a pigment-depositing gene (e), it can only have yellow coat color. In a cross of two doubly heterozygous black dogs (BbEe x BbEe), what fraction of the next generation would we expect to be yellow? WHY?

a.1/8

b.1/4

c.2/3

d.3/16

Answer 1

Answer - b. 1/4.

Explanation - The cross is represented as follows.

Parents - Black male x Black male

Genotype - BbEe x BbEe

Gametes - BE, Be, bE, be x BE, Be, bE, be

F1 progeny -   

  

Gametes	BE	Be	bE	be
BE	BBEE black	BBEe black	BbEE black	BbEe black
Be	BBEe black	BBee yellow	BbEe black	Bbee yellow
bE	BbEE black	BbEe black	bbEE brown	bbEe brown
be	BbEe black	Bbee yellow	bbEe brown	bbee yellow

The pigment deposition gene 'e' in recessive homozygous condition (ee) produces yellow colour. In the F1 progeny, 4 individuals have 'ee' condition out of the 16 offspring. So the number of offspring with yellow colour is 4/16 which is equialent to 1/4. So the answer is option b. 1/4.

The inheritance shown here is Recessive Epistasis. In recessive epistasis, homozygous recessive allele of a gene mask the expression of a gene in another location. The recessive epistasis produce a 9 : 3 : 4 ratio.  

Here black coat colour allele B is dominant over brown coat colour allele b. The homozygous or heterozygous condition of B (BB or Bb) produce black coat colour. The homozygous recessive condition (bb) produces brown coat colour. But when pigment deposition gene E is present in homozygous recessive condition (ee), it mask the expression of black and brown coat colour. The offspring with ee have yellow coat colour. The homozygous recessive ee is epistatic to coat colour geneB. So it shows Recessive epistasis inheritance.