Question

in labrador retrievers, black hair pigment (b) is dominant to brown hair pigment (b); e allows pigment to be deposited in the hairs, while ee individuals deposit no pigment in their hair and appear yellow. if two black labs mate and their litter includes black, brown, and yellow puppies, what are the genotypes of the two black labs? Draw a punnett square showing the gametes possible from these black labs andthe genotypes expected in their offsprings. Assume independent assortment for these genes.

Answer 1

If two black labs mate and the litter has black, brown and yellow labs, then the two must have been heterozygous at each locus. One of the tricks in this question is that ee will produce a yellow lab, no matter what alleles the B gene has (in a sense the ee genotype overrides whatever the B gene genotype is, whether it is BB, Bb or bb). So, the labs must have a B allele to be black and cannot be bb (which would be brown) but must have a b allele to pass on to the offspring so that there can be brown labs in the offspring, so the labs must be Bb. Further, since the labs are black, neither can be ee and must have at least one E allele. However, since there are yellow labs in the offspring, then there must be an e allele in each lab, so their genotype is Ee. Thus, each of the labs is BbEe. So, since this is a dihybrid cross, there will be 16 possibilities (16 squares in the Punnett Square). The cross is:

BbEe x BbEe

The offspring from a dihybrid cross usually come out in a 9:3:3:1 ratio, but the ee locus will make things a little different:

B-E- 9 black lab
B-ee 3 yellow lab
bbE- 3 brown lab
bbee 1 yellow lab

So, the ratio is actually 9 black labs : 3 brown labs : 4 yellow labs.

The complete list of possible genotypes from the cross is (note that there are 16 possibilities, some of which repeat and you will note that there are 9 black, 3 brown and 4 yellow):

BbEe x BbEe

offspring
BBEE black
BBEe black
BBEe black
BBee yellow

BbEE black
BbEe black
BbEe black
Bbee yellow

BbEE black
BbEe black
BbEe black
Bbee yellow

bbEE brown
bbEe brown
bbEe brown
bbee yellow