Expert: Walter Hintz - 12/6/2005

Question
A couple who are both carriers of the gene for cystic fibrosis have two children who have cystic fibrosis.  What is the probability that their next child will have cystic fibrosis?A.50% B.75%
C. 25% D. 100% e. 0%
And how do you figure it out?
Thank you!!!

Answer
The trait is recessive. With two parent carriers the chance of having a child with it is 25%.
 Make a punnett square
 Let C =normal
     c= Cystic fibrosis
   C              c

C  CC              Cc

c   Cc            cc (disease)
  
 Three normal and one with it.
 Odds are 1 in 4
 What the first two children have has no influence on the third