You have two dogs: a brown Labrador retriever and its sister, who is black. In these dogs, the B allele (black) is dominant over the b allele (brown). A bb dog is brown. Which of the following parent genotypes could NOT have resulted in the litter to which both of your dogs belong?

A. Mother BB ~ Father Bb
B. Father Bb * Mother Bb
C. Mother Bb * Father Bbm
D. It cannot be determined from the information given.

A. Mother BB ~ Father Bb

Explanation:

Given, black (B) is dominant over brown (b). So,

BB and Bb = Black

bb = brown

It is given that one sibling is black (BB/Bb) and another sibling is brown (bb). To produce a brown offspring both the parents should have at least one b allele.

If mother is BB and father is Bb, a brown offspring can not be produced because the mother doesn't have any b allele.

BB X Bb:

     B      B

B  BB   BB

b  Bb    Bb

All the offspring will be black because they all have at least one B allele.

Hence, Mother BB ~ Father Bb genotype can not produce this litter.

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