A yellow female Labrador retriever was mated with a brown male. Half of the puppies were brown, and half were yellow. Explain how the same female, when mated with a different brown male, could produce only brown offspring.
O The firsf male was bb Ee, and the second male was bb ee.
O The first male was bb Ee, and the second male was Bb Ee.
O The first male was bb Ee, and the second male was bb EE.
O The first male was bb EE, and the second male was bb Ee.
O The first male was Bb EE, and the second male was Bb Ee.

In Labradors coat colour is controlled by two genes. Suppose the two genes are B and E. B produces black colour and recessive form bb gives brown colour. Gene E is epistatic over gene B in its recessive form which means that ee will produce yellow colour regardless of the genotype present of B gene.

The first case is possible if the female lab is bbee (yellow) and the male lab is bbEe (brown):

bbee X bbEe

bE be

be bbEe bbee

So half of the offspring will be brown (bbEe) and half of them will be yellow (bbee)

The second case is possible if the same female bbee mates with a brown male of different genotype which can be bbEE:

bbee X bbEE

bE

be bbEe

So all offspring will be brown (bbEe)

Hence, the first male was bbEe and the second male was bbEE.