Question

The CFTR gene is carried on human chromosome 7, an autosome. A recessive mutation in CFTR is the cause of cystic fibrosis disease. A female who doesn't have cystic fibrosis has a child with a man who also doesn't have the disease. Their first child has cystic fibrosis. What is the probability their second child will have the disease?

Answer 1

ANSWER: For the child to have Cystic fibrosis, child must contain 2 copies of cystic fibrosis gene with mutation.

• For the child to inherit CF, either both the parents must have the disease or they must be carriers of the disease.
• Since it's already given in the above scenario that the parents don't have CF therefore, they must be CARRIERS FOR THE DISEASE.
• As the parents are carriers, so the first child must have inherited one copy of mutated CFTR from each parent.
• Considering parents to be carriers: There is 25% possibility of second child having cystic fibrosis.
• Overall possibilities: 25% possibility of a diseased child, 50% possibility of carriers and 25% possibility of a normal child i.e., without Cystic Fibrosis.