Question

A pedigree shows that two parents are phenotypically normal, but one of their four biological children has a typical autosomal recessive trait. The other three children are phenotypically normal. It is very likely that:

a) Both parents are carriers.
b) One parent is a carrier.
c) Both parents are homozygous recessive.
d) This is a random occurrence with no genetic basis.

Answer 1

If a child inherits an autosomal recessive disease and their siblings are normal, the most likely scenario is that both parents are carriers (Aa) of the disease allele, fitting a 3:1 ratio of unaffected to affected offspring.

Step-by-step explanation:

The question revolves around the inheritance pattern of an autosomal recessive trait. If two parents are phenotypically normal but have one child with an autosomal recessive disease, the most likely explanation is that both parents are carriers. This means that each parent has one normal allele and one mutant allele (Aa). When both carriers have children, there is a 25% chance that any given child will be affected (aa), a 50% chance that the child will be a carrier (Aa), and a 25% chance that the child will have normal alleles (AA).

Because one of their four children has the condition, and the other three are normal, it fits the expected 3:1 ratio for autosomal recessive inheritance with two carrier parents. Therefore, option a) Both parents are carriers is the most likely answer.