As a Drosophila research geneticist, you keep stocks of flies of specific genotypes. You have a fly that has normal wings (dominant phenotype). Flies with short wings are homozygous for a recessive allele of the wing-length gene. You need to know if this fly with normal wings is pure-breeding or heterozygous for the wing-length trait. What cross would you do to determine the genotype, and what results would you expect for each possible genotype?

You have to do a test cross. If all the offspring have normal wing, then the the stock are from pure-breeding flies, but if 3/4 of the offspring have normal wings and 1/4 have short wings, then the stock is from heterozygous for the wing-length trait.

Explanation:

A test cross was proposed by Mendel. It was developed to determine the zygosity of an individual. It consists in crossing the individual (which present the dominant phenotype) with an individual that exhibits the recessive phenotype. Then you have to analyze the proportions of phenotypes in the offspring.

If a pure-breeding individual is crossed with a homozygous recessive individual; then all the offspring will have the dominant phenotype.

If a heterozygous individual is crossed with a homozygous recessive individual; then 3/4 of the offspring will have the dominant phenotype, and 1/4 will have the recessive phenotype.

To have the dominant phenotype, the individual need to have at least one dominant allele, while to have a recessive phenotype, both alleles need to be recessive.