Blue eyes are an autosomal recessive trait. A blue eyed son is born to a brown-eyed mother and a blue-eyed father. If these parents have two more children, both with blue eyes, what is the probability that their fourth child will have brown eyes?
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The fact that these parents have had any children with Blue Eyes means that the mother had a blue-eyed parent so that she carries the recessive genes for blue eyes. Beyond that fact, the history of blue-eyed children has no bearing on the likelihood of the eye color of future offspring.
The best tool for solving this is a Punett Square
b Bb bb
b Bb bb
The Capital Bs are the dominant trait of Brown eyes, and the lower-case bs are the recessive trait of blue-eyes. The top is the Mother's hybrid Genotype for Brown eyes, and the left side is the pure-bred blue-eyed Father. The pairings are the Genotypes that their children could have. 2/4 or 1/2 would have brown eyes, and 2/4 or 1/2 would have blue eyes.
So the probability of the 4th Child having Brown eyes is 1 out of 2, or 1/2.
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