Question

A breeder of black Labrador puppies notices that in his line a gene predisposing his dogs for white spots has arisen. He believes the white spot allele is autosomal recessive, and he wants to prevent it from continuing in his dogs. He sees that one of his female dogs, Speckle, has white spots, and he therefore assumes she is homozygous for the gene. But he must determine which males in his pack are heterozygous to avoid breeding them in the future. What is a reasonable plan? (A) Stop breeding Speckle. (B) Pair Speckle with a male with white spots. (C) Perform a testcross of Speckle with a black Labrador male. If any pups are born with white spots, stop breeding that male because he is heterozygous. (D) Breed Speckle with a chocolate Labrador.

Short answer

The best plan is to perform a testcross of Speckle with a black Labrador male. If any pups are born with white spots, stop breeding that male because he is heterozygous. (C)

Step-by-step solution

Understanding the given options

It's important to first understand the given options and their implications: (A) Stop breeding Speckle: This option suggests that by not breeding Speckle, the issue of spreading the white spot allele can be prevented. However, this would not help in identifying heterozygous males. (B) Pair Speckle with a male with white spots: This option will mate Speckle with another dog showing the white spot trait. This may perpetuate the white spot allele but does not address the concern of identifying heterozygous males. (C) Perform a testcross of Speckle with a black Labrador male: If any pups are born with white spots, stop breeding that male because he is heterozygous. This option suggests using a testcross to identify heterozygous males. (D) Breed Speckle with a chocolate Labrador: This option suggests breeding with a different color Labrador. However, it doesn't specifically help in identifying heterozygous males or preventing the spread of the white spot allele.

Choosing the best option

Option C is the best choice because it proposes a focused and logical way to identify heterozygous males, which was the main concern of the breeder. A testcross is performed with a black Labrador male, as they cannot have white spots (meaning they have at least one dominant allele). If they produce offspring with white spots, it means that the male carries the recessive white spot allele (heterozygous), and the breeder should avoid breeding that male. Therefore, the best plan is: (C) Perform a testcross of Speckle with a black Labrador male. If any pups are born with white spots, stop breeding that male because he is heterozygous.