Final answer:
A test cross is used to determine the unknown genotype of an individual expressing a dominant phenotype by crossing them with a homozygous recessive individual, revealing whether the original parent is homozygous or heterozygous dominant based on the offspring's phenotypes.
Step-by-step explanation:
To determine the unknown genotype of an individual with a dominant phenotype, a test cross can be utilized. This type of cross involves breeding the individual in question with a homozygous recessive individual. The resulting offspring phenotypes can then reveal the unknown parent's genotype. If the dominant-expressing organism is homozygous for the dominant trait (AA), all the F1 offspring will express the dominant trait, as they will all inherit at least one dominant allele (Aa). Alternatively, if the dominant-expressing organism is heterozygous (Aa), the offspring will show a 1:1 ratio of dominant (Aa) to recessive (aa) phenotypes, as some will inherit a recessive allele from both parents.