Final answer:
Two and three-factor testcrosses are used to determine if genes assort independently and to identify homozygous recessive individuals. This clarifies whether the dominant-expressing individual is homozygous or heterozygous for a trait.
Step-by-step explanation:
In addition to determining genotypes, two and three-factor testcrosses are used to determine if two genes assort independently and to identify homozygous recessive individuals in the F2 generation. A testcross involves crossing a dominant-expressing individual with an unknown genotype with a homozygous recessive individual. This can reveal whether the dominant-expressing parent is homozygous or heterozygous for the dominant trait.
If the dominant-expressing organism is a homozygote, all F1 offspring will be heterozygotes expressing the dominant trait. However, if the dominant-expressing organism is a heterozygote, the F1 offspring will display a 1:1 ratio of heterozygotes to recessive homozygotes, helping to determine the mode of inheritance and validates Mendel's principle of segregation.