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MCAT Question A Day - 9/22/14 - Answer!

Color-blindness is a sex-linked recessive trait. A woman who is a carrier for the trait has two boys with a color-blind man. What is the probability that both boys are color-blind?

A.  0%
B.  25%
C.  50%
D.  100%

B is correct. According to the Punnett square, each time they have a boy, there is a 50% chance that he will be color-blind. The chance that both boys are colorblind is the chance of this happening twice, or (0.5)2.

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