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?
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.