In labrador dogs coat color is determined by alleles at the

In labrador dogs, coat color is determined by alleles at the E and B genes. The following genotypes cause the coat colors shown:

E-B-      black
E-bb      brown
eeB-      yellow
eebb      yellow

In a cross between two black labradors, both with genotype EeBb, which is the ratio of black, brown and yellow puppies expected in the offspring:

a.

9:3:3:1

b.

12:3:1

c.

9:6:1

d.

9:3:

Solution

The coat color in Labradors is controlled by 2 genes, E gene and B gene. It is an example of epistasis amongst the genes implying that the 2 genes interact to give the resulting coat color which may be black, brown or yellow. The E gene is dominant epistatic because if it is present the coat would have dark pigmentation. The presence of B gene in dominant form gives a higher degree of pigmentation. So, when E gene is present with B gene, it would give black color (Homozygous and heterozygous dominance, both work). When the E gene is present with recessive b gene, the degree of pigmentation would be less and result in brown coat. But when E gene is recessive i.e. in ee form, no matter what form the b gene is in, it would not give dark pigments resulting in a yellow coat color. So when we cross EeBb x EeBb, we get 9 E_B_: 3 E_bb: 3 eeB_: 1eebb.

In the 4 cases where ee is present we would get a yellow coat. In the 3 cases where bb is present we would get a brown coat. The 9 cases of E_B_ would result in a black coat.

So, the ratio would be option d) 9:3:4