Final answer:
A test cross is more efficient with a homozygous recessive donor as it directly reveals the genotype of the unknown parent, showing whether it is homozygous dominant or heterozygous based on the offspring's phenotypes. The same can't be as easily discerned with a homozygous dominant donor, as all offspring would display the dominant trait, offering no insight into the parent's genotype without additional crosses.
Step-by-step explanation:
Performing a test cross with a homozygous recessive donor is more efficient than using a homozygous dominant donor because it directly reveals the genotype of the unknown parent. If the unknown is a heterozygote, the offspring will show a 1:1 ratio of dominant to recessive phenotypes. On the other hand, if it's a homozygous dominant, all offspring will exhibit the dominant trait.
The reason for doing a test cross, generally, is to identify heterozygous individuals with the dominant phenotype or to determine allele dominance. While getting the same information with a homozygous dominant donor is not impossible, it is more complicated and less straightforward. If the unknown genotype individual was crossed with a homozygous dominant, all offspring would show the dominant trait, giving no information about the unknown parent's genotype. To ascertain the genotype, one would then have to perform test crosses of the offspring to look for the appearance of the recessive trait.
Mendel's method of the test cross is still employed today by plant and animal breeders to distinguish between homozygous dominants and heterozygotes when organisms express a dominant trait. It leverages the principle that homozygous recessive individuals will only pass on the recessive allele, thereby providing clear insight into the genotype of the dominant-expressing organism when analyzing the offspring's phenotype.