Question 13 A child is born with cystic fibrosis, an inherited recessive disorder. No one in the family (on either side) for several generations is known to carry the gene or have the disorder. How can this be explained? a The genetic disorder formed from the mother's poor nutrition and prenatal care. b There was a mutation in the DNA, either in one of the gametes or the fertilized egg. c The child contracted the illness from another child when he was born. d The child developed the disorder when he was born; it is a result of a birth defect.

Answer 1

The most accurate explanation for a child being born with cystic fibrosis when no one in the family has the disorder or is known to carry the gene would be:

b. There was a mutation in the DNA, either in one of the gametes or the fertilized egg.

Cystic fibrosis is caused by mutations in the CFTR gene, and if neither parent shows symptoms or has a family history of the disorder, it is possible that one or both parents could be carriers (asymptomatic) or that a new mutation occurred in the child’s DNA. This is a relatively rare scenario, but it is a valid explanation for the appearance of an inherited recessive disorder.