Final answer:
In a test cross, an organism with the dominant phenotype is crossed with an organism that is homozygous recessive for the same characteristic. This can help determine whether an individual is homozygous or heterozygous.
Step-by-step explanation:
A test cross can be used to determine whether an individual is homozygous or heterozygous. In a test cross, an organism with the dominant phenotype is crossed with an organism that is homozygous recessive for the same characteristic. If the dominant-expressing organism is a homozygote, then all F1 offspring will be heterozygotes expressing the dominant trait. Alternatively, if the dominant expressing organism is a heterozygote, the F1 offspring will exhibit a 1:1 ratio of heterozygotes and recessive homozygotes.