Question

In dogs, there is a hereditary deafness caused by a recessive allele, d. A kennel owner has a male dog that she wants to use for breeding purposes if possible. The dog can hear, but the owner is unsure of the genotype. She does a testcross (crosses it to a homozygous recessive dog), and two of the five offspring are deaf. This means that the male dog

A. is actually deaf
B. has the genotype Dd
C. is still of unknown genotype since there were offspring of both deaf and hearing phenotypes
D. has the genotype dd
E. has the genotype DD

Answer 1

The male dog can hear and has a heterozygous genotype (Dd) because it produced both hearing and deaf offspring when crossed with a homozygous recessive dog.

Step-by-step explanation:

The question inquires about the genotype of a male dog used in a testcross where the offspring show a mix of hearing and deaf phenotypes. Since the male dog can hear, it must have at least one dominant allele (D), so its genotype cannot be dd. The fact that two of the five offspring are deaf (and hence must have the genotype dd) indicates that the male dog provided a recessive allele (d) to these puppies. Therefore, the male dog's genotype must be heterozygous (Dd), as it contributed a recessive allele to the deaf offspring and a dominant allele to the hearing offspring.