What technique is used to determine the genotype of an organism with a dominant phenotype?

The technique used to determine the genotype of an organism with a dominant phenotype is a test cross. This method involves breeding the organism in question, which shows the dominant trait, with another organism that is homozygous recessive for the same trait. Since the recessive phenotype is expressed only when the organism has two recessive alleles, the offspring produced from this cross can provide clarity on the genotype of the dominant phenotype organism.

If the dominant organism is homozygous dominant, all offspring will display the dominant phenotype. If the organism is heterozygous, the offspring will show a mix of phenotypes—some exhibiting the dominant trait and others displaying the recessive trait. This pattern of inheritance provides critical insight into the genotype of the dominant phenotype individual. This is a reliable method for ascertaining whether the organism carries one or two alleles for the dominant trait.