Dr. Smith's parents have normal hearing. However, Dr. Smith has an inherited form of deafness. Deafness is a recessive trait that is associated with the abnormal allele d. The normal allele at this locus, associated with normal hearing, is D. Dr. Smith's parents could have which of the following genotypes?
A) DD and dd
B) dd and dd
C) Dd and Dd
D) Dd and DD

Question

Quizplus · Accepted Answer

Since Dr. Smith has an inherited form of deafness, he/she must have inherited the recessive allele d from both parents. Therefore, both parents must be carriers of the deafness allele, which means they must have one normal allele (D) and one deafness allele (d). The only genotype that satisfies this condition is Dd. Therefore, the parents must be heterozygous (Dd) carriers of the deafness allele. Option C represents this possibility. The other options, A, B, and D, either have one parent with two copies of the normal allele (DD) or both parents with two copies of the deafness allele (dd), neither of which is possible if Dr. Smith inherited the form of deafness.

Dr Smith's Parents Have Normal Hearing