When a gray fly was crossed with a black fly, all the offspring had a gray color.

When a gray fly was crossed with a black fly, all the offspring had a gray color. Determine the genotype of the gray fly (gray is the dominant trait).

When a gray fly was crossed with a black fly, all the offspring had a gray color, and the gray color of the fly’s body was the dominant feature. This is a classic example of Mendel’s first law: when individuals with opposite traits are crossed, only one of a pair of traits appears in the first generation. Mendel called this feature dominant.
Let A be a gray color and a black color. The recessive gene manifests itself only in a homozygous state, which means that the genotypes of the parents are AA and aa. Gray offspring have the Aa genotype.