In a genetic cross between two true-breeding lines—one with sepia-eyed flies and one with wild-type eyes—if sepia is autosomal recessive, what percent of the F2 generation is expected to have sepia eyes?

• A. 50%
• B. 25%
• C. 0%
• D. 75%

Answer: (B)

Autosomal recessive traits require two copies of the recessive allele to show the phenotype. Crossing true-breeding lines where one parent is sepia (homozygous recessive, ss) and the other is wild-type (homozygous dominant, SS) gives F1 offspring that are all heterozygous (Ss) and display the wild-type phenotype. When F1 individuals mate (Ss × Ss), the F2 genotypes segregate in a 1:2:1 ratio (SS : Ss : ss), which translates to a 3:1 phenotypic ratio of wild-type to sepia. So 1 out of 4, or 25%, of the F2 generation are expected to have sepia eyes. Being autosomal means this pattern is the same in males and females.