SAT Subject Biology Practice Question 24: Answer and Explanation
1. In fruit flies, brick-red eye color is dominant to a bright orange color called cinnabar. A fruit fly with heterozygous eye color geno-type is crossed with homozygous recessive fruit fly. What is the probability of inheritance for the cinnabar phenotype and the heterozygous genotype?
A. Cinnabar phenotype = 1.0; Heterozygous genotype = 0
B. Cinnabar phenotype = 0; Heterozygous genotype = 1.0
C. Cinnabar phenotype = 0.5; Heterozygous genotype = 0.5
D. Cinnabar phenotype = 0.25; Heterozygous genotype = 0.5
E. Cinnabar phenotype = 0.5; Heterozygous genotype = 0.75
Correct Answer: C
The Punnett square for the cross is shown below. Each offspring has a 50% chance of inheriting a heterozygous phenotype and an equal chance of inheriting a homozygous recessive genotype (which results in the cinnabar phenotype).