MCAT Question A Day - 4/30/13 - Answer!
A test cross of a black guinea pig male produced six black male offspring and two brown female offspring. If black is dominant to brown, what is the genotype of the male parent?
A. X(B)Y
B. X(b)Y
C. Bb
D. BB
The correct answer is (C). A test cross is performed to determine the genotype of an individual using a double recessive partner. Since both black and brown offspring were produced, the black male guinea pig parent must have been Bb. If he had been BB (choice D), all offspring would have been black. If coat color was a sex-linked trait, no black male offspring could be produced by a brown mother and black father (choice A). Choice (B) X(b) is incorrect because that is the genotype of a brown guinea pig father.
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