Expert: Walter Hintz - 11/26/2007

Question
Here is the question i was given: A man with hemophilia (a recessive, sex-linked condition)has a daughter who is of a normal phenotype. She marries a man who is normal for the trait. What is the probability that a daughter of this mating will be a hemophiliac?Or a son who is hemophiliac? If they have 4 sons, what is the probability that all 4 will be born with hemophilia? i do not understand how to get the probability.

Answer
Hi Johanna
Lets make a punnett square
   Let X  = normal gene
   Let Xh = hemophilia
   
   Let XX  = Normal female  and XXh = female carrier
   Let XY  = Normal male  and XhY = hemophiliac male

  The daughter in the problem must be XXh since her father was XhY
   She marries a normal Male
       XXh       x     XY

       X             Xh

  X   XX            XXh

  Y   XY            XhY

  Chance is that

    1/2 of daughters are carriers: none have hemophilia
    1/2 of sons have hemophilia

Out of 4 sons 2 would have hemophilia