The test cross is another one of the fundamental tools devised by Gregor Mendel.  In its simplest form this is an experimental cross of an individual organism of dominant phenotype but unknown genotype to an organism with a  homozygous recessive genotype (and phenotype).

Example for a Single Gene.

In the fruit fly Drosophila melanogaster  black body (b) is recessive to the normal gray body (B).  You are given a male with a gray body.  Determine its genotype.

Procedure : The male has one of two genotypes : BB or Bb.  This male is crossed to a black-bodied female of genotype bb.  There are two possible outcomes:

In outcome 1, the male had the genotype Bb.  By the principle of segregation,  he made two gametes B and b in equal frequencies.  The tester female had genotype bb and made only one gamete (b) with a frequency of 1. The progeny of this cross were 50% gray body and 50% black body reflecting the type and frequency of their father's gametes.

In outcome 2, the male had the genotype BB.  He made only one gamete (B) with a frequency of 1.   The tester female had genotype bb and made only one gamete (b) with a frequency of 1. The progeny of this cross were 100% gray heterozygotes (Bb) reflecting the type and frequency of their father's gametes.