What is the primary purpose of a testcross in genetics?

The primary purpose of a testcross is to reveal the genotype of an individual exhibiting a dominant phenotype. When you encounter an organism that shows a dominant trait, it can be either homozygous dominant (having two identical alleles for that trait) or heterozygous (having one dominant and one recessive allele). A testcross involves breeding this individual with a homozygous recessive individual for the same trait. By observing the phenotypic ratios of the offspring, it's possible to infer the genotype of the unknown parent. If all offspring display the dominant trait, the unknown parent is likely homozygous dominant. If there is a mix of both dominant and recessive traits among the offspring, the unknown parent is likely heterozygous.

This technique is fundamental in genetics as it provides a clear method of determining genotype based on observable traits in progeny. Understanding the genotype of an organism helps in various areas of genetic research, breeding, and understanding inheritance patterns.