What is the purpose of a test cross?

A test cross is conducted primarily to determine the genotype of an organism that expresses a dominant phenotype. This situation arises because dominant traits can result from either a homozygous dominant genotype (two dominant alleles) or a heterozygous genotype (one dominant and one recessive allele). By crossing the organism in question with a known homozygous recessive individual—who can only contribute recessive alleles—the offspring produced can provide clear insights into the genotype of the dominant phenotype parent.

If any offspring exhibit the recessive phenotype, it indicates that the dominant phenotype parent is likely heterozygous. Conversely, if all offspring show the dominant phenotype, this suggests that the parent is likely homozygous dominant. This method is vital for geneticists in breeding experiments, as understanding the genotype is crucial for predicting traits in future generations.

The other choices involve different concepts in genetics. Determining a phenotype by examining the effects of mutations is not focused on identifying underlying genotypes. Examining allele behavior over multiple generations does not specifically address the issue at hand, which is determining the unknown genotype of a dominant trait.