Question

family tree – cystic fibrosis
woman with cystic fibrosis man with cystic fibrosis
woman without cystic fibrosis man without cystic fibrosis
rob jane
leon claire owen shane paula keith
sian david laura
how could shane have cystic fibrosis if neither of his parents and none of his siblings have the disease?
both of his parents must have the homozygous genotype.
both of his parents must have the heterozygous genotype.
both of his parents must have the cystic fibrosis phenotype.

Explanation:

Cystic fibrosis is an autosomal recessive genetic disorder, meaning a person must inherit two copies of the defective recessive allele (homozygous recessive) to have the disease. Unaffected parents can have an affected child if both parents carry one copy of the defective allele (heterozygous genotype) – they do not show symptoms but can pass the recessive allele to their offspring. When two heterozygous parents have a child, there is a 25% chance the child inherits two recessive alleles and develops the disease. The other options are incorrect: homozygous parents would either all be affected (if homozygous recessive) or unable to pass the recessive allele (if homozygous dominant), and parents with the cystic fibrosis phenotype would themselves have the disease, which contradicts the question's premise.

Answer:

Both of his parents must have the heterozygous genotype.