USA Biology Olympiad (USABO) Practice Exam

In a genetic scenario involving fur color in mice, what is the probability that offspring from two gray mice will not be white?

1. 1/2

2. 1/4

3. 3/4

4. 2/4

The correct answer is: 3/4

To determine the probability that offspring from two gray mice will not be white, it is essential to understand the underlying genetics of fur color in these mice. Typically, fur color can be influenced by a few alleles, where gray is often dominant over white. If we assume that the gray mice can each carry either a homozygous dominant genotype (GG) or a heterozygous genotype (Gg), and white is represented by the homozygous recessive genotype (gg), we can assess their potential offspring. When two gray mice breed, the possible combinations of genotypes for their offspring will depend on the parents' genotypes. If both gray mice are homozygous (GG), all offspring would be gray (100% gray). If one is homozygous (GG) and the other heterozygous (Gg), then 50% of the offspring would be gray (GG or Gg) and the other 50% would also be gray, leading to no offspring being white. However, if both parents are heterozygous (Gg), the resulting genotypic ratio for the offspring would be 1 GG : 2 Gg : 1 gg, giving a probability of 1/4