Question

Early-onset Alzheimer's (A) is caused by a rare autosomal dominant allele that accounts for 3% of all cases of Alzheimer's. What can you conclude about the woman's mother and father?

a) Both parents are carriers for A
b) Only the mother is a carrier for A
c) Neither parent is a carrier for A
d) Both parents have Alzheimer's

Answer 1

A woman with early-onset Alzheimer's must have inherited the autosomal dominant allele from at least one parent, but neither parent necessarily exhibits Alzheimer's disease. One parent is a carrier of the allele.

Step-by-step explanation:

If a woman has early-onset Alzheimer's caused by an autosomal dominant allele, it is enough if one parent has the allele for her to inherit it. Hence, neither parent needs to have Alzheimer's, but at least one of them must be a carrier. There's a 50% chance that a child of a carrier will inherit the allele and therefore the potential to develop the disease. Given that it is a rare allele accounting for 3% of all Alzheimer's cases, it is more likely that one parent is a carrier rather than both. So, It is not possible to conclude that both parents have Alzheimer's, but at least one must carry the autosomal dominant allele.

Considering that the disease is caused by an autosomal dominant allele which is rare, indicates that the mother or father could pass on the allele even if they do not exhibit symptoms themselves due to incomplete penetrance or late-onset, where the parent has not yet shown symptoms. Moreover, since Alzheimer's can also occur as sporadic with no clear genetic cause or could be influenced by multiple genetic and environmental factors, the exact genetic status of the parents requires genetic testing to confirm.