Final answer:
The risk that an unaffected full sibling of a patient with cystic fibrosis carries a mutated CF gene is 2 in 3, as two out of three possible outcomes for an unaffected sibling in a family with two carrier parents are carrier genotypes.
Step-by-step explanation:
The risk that an unaffected full sibling of a patient with cystic fibrosis carries a mutated CF gene is 2 in 3 (Option D). Given that cystic fibrosis is an autosomal recessive condition, if one sibling is affected (has cystic fibrosis), it means both parents are carriers of the faulty gene. The Punnett square for two carrier parents (Ff) shows that there is a 25% chance for a child to have cystic fibrosis (ff), a 50% chance for a child to be a carrier (Ff), and a 25% chance for a child to have two normal alleles (FF).
Therefore, there are three possible genotypes for the sibling (cf. carrier, cf. carrier, non-carrier), but since we know the sibling is unaffected, we exclude the affected scenario. Out of the two remaining genotypes (carrier and non-carrier), carriers make up 2 of the 3 possible outcomes for an unaffected sibling.