Question

A black male mouse (I) is crossed with a black female mouse, and they produce 20 offspring; 15 black and 5 white. A different black male mouse (II) is crossed with the same female, and the offspring from this mating are 30 black mice. Which of the following must be true?

A) Mouse I carries a recessive white allele.
B) Mouse II carries a dominant black allele.
C) Mouse I carries a dominant black allele.
D) Mouse II carries a recessive white allele.

Answer 1

Based on the results of the two crosses, we can confirm that Mouse I carries a recessive white allele as white offspring were produced. For Mouse II, only the presence of a dominant black allele is certain as all offspring were black.The correct option is A.

Step-by-step explanation:

The question revolves around understanding inheritance patterns, particularly with a focus on epistasis in mice. From the information provided and the breeding results, we can infer the following:

1. Since the first cross with the black male mouse (I) and the black female resulted in both black and white offspring, it is likely that both parents carry a recessive allele for white coat color. This means that Mouse I carries a recessive white allele because the presence of white offspring suggests that two recessive alleles came together.
2. Considering that the second cross with a different black male mouse (II) resulted in all black offspring, Mouse II must carry at least one dominant black allele. However, we cannot categorically determine the genotype (whether it is homozygous or heterozygous) without further information.

Based on this analysis, the correct answer must be true:
A) Mouse I carries a recessive white allele. As for Mouse II, we can only conclude that it carries a dominant black allele, but further crosses would be required to determine if it also carries a recessive white allele.The correct option is A.