SAT Subject Biology Practice Question 390: Answer and Explanation
20. Black coat color in horses is caused by a dominant allele, while white coat color is due to the recessive allele. Two black horses produce a foal with a white coat. If they were to produce a second foal, what would be the probability of the second foal having a black coat?
Correct Answer: D
D Again, make sure you read the question carefully and know exactly what you're being asked. If black coat color in horses is the dominant phenotype, and both parental horses are black, then they must have at least one allele for black coat color. Because white coat color is the recessive phenotype, it can be caused only by a homozygous recessive genotype. The fact that the parental horses produced a white foal indicates that they must BOTH be heterozygous for coat color, each having an allele for white coat color as well as black. In the Punnett square below, B represents the dominant allele (causing black coat color) and b represents the recessive allele (causing white coat color). The probability of these horses producing a foal with a black coat is 75%, or 3 out of 4.
Note that the fact that we're asked about the second foal produced doesn't really matter. Each time the parental horses mate they have the same probability of producing black foals (75%) or white foals (25%). The coat color of previous offspring does not change the probability.