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Genetic Counselor

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Real-Life Math

Cystic fibrosis (CF) is a genetic disease with an autosomal recessive inheritance pattern. "Autosomal" means that the specific gene responsible for CF is located on a chromosome other than the 2 sex chromosomes. Therefore, its inheritance is not related to gender.

"Autosomal recessive" means that in order for a person to be affected by CF, she or he must have both copies of the mutated CF gene.

This means that one copy of the mutated gene was inherited from the father and the other copy was inherited from the mother. Therefore, a person who is affected by CF has parents who each have 1 normal copy of the CF gene and 1 copy with a mutation. (This assumes that neither parent has CF, which is the case most of the time).

For such a couple, in each pregnancy the sperm may have a normal CF gene or a mutated gene. The egg may have a normal CF gene or a mutated gene.

  1. How many different possible combinations are there for each pregnancy (for this couple)? What is the probability that their child will have CF?
  2. If 2 people with the CF gene have 3 children, what is the probability that all 3 will have cystic fibrosis?
  3. If a couple's 1st child is born with cystic fibrosis, what is the probability their 2nd child will be born with it?