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Blue eyes are an autosomal recessive trait.  A blue eyed son is born to a brown-eyed...

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motherof4babies | eNotes Newbie

Posted April 7, 2013 at 9:40 PM via web

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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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tjbrewer | Elementary School Teacher | (Level 2) Associate Educator

Posted April 7, 2013 at 10:55 PM (Answer #1)

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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    b
   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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