What is a test cross and why is it used?

A test cross is a genetic breeding experiment used to determine an organism's genotype.

In more detail, a test cross is a method used in genetics to determine whether an organism exhibiting a dominant trait is homozygous dominant (AA) or heterozygous dominant (Aa). This is achieved by crossing the organism with an organism that is homozygous recessive (aa) for the trait in question. The test cross is a fundamental tool in the study of genetics, and it is used to establish the genotype of an organism when the phenotype alone cannot provide this information.

The principle behind a test cross is simple. If the organism being tested is homozygous dominant, all offspring produced will exhibit the dominant trait, as they will all receive at least one dominant allele. However, if the organism is heterozygous dominant, approximately half of the offspring will exhibit the recessive trait, as they have a 50% chance of inheriting the recessive allele.

For example, in pea plants, the allele for yellow seeds (Y) is dominant over the allele for green seeds (y). If a plant with yellow seeds is crossed with a plant with green seeds, and the offspring all have yellow seeds, the yellow-seeded parent is likely homozygous dominant (YY). If some of the offspring have green seeds, the yellow-seeded parent is heterozygous dominant (Yy).

In summary, a test cross is a crucial tool in genetics, allowing scientists to determine the genotype of an organism based on the traits exhibited by its offspring when crossed with a homozygous recessive organism. It is a fundamental concept in the study of inheritance and genetic variation.