Under the law of dominance in genetics , an individual expressing a dominant phenotype may have two copies of the dominant allele ( homologous dominant) or one copy of each dominant and recessive allele ( heterozygous dominant).  By doing a test cross, one can determine whether an individual is homozygous or heterozygous to be dominant.
In a test cross, the individual in question is bred with another individual who is homozygous for the recessive trait and the test cross’s offspring are screened.  Since a homozygous recessive individual can only pass on recessive alleles, the alleles considered determine the phenotype of the offspring.  Thus, this test gives rise to 2 possible situations:
- If any of the offspring produced express the recessive trait, then the individual in question is heterozygous for the dominant allele. 
- If all offspring produced express the dominant allele, then the individual in question is homozygous for the dominant allele. 
The first use of the test cross was in Gregor Mendel ‘s experiments in hybridization of plants . Studying the inheritance of dominant and recessive traits in pea plants, they show that an individual’s “meaning” (now called zygosity ) for a dominant trait is determined by the expression patterns of the following generations. 
The rediscovery of Mendel’s work in the early 1900s led to an explosion of experiments employing the principles of the test cross. From 1908–1911, Thomas Hunt Morgan conducted test crosses to determine the inheritance pattern of a white eye color mutation in Drosophila.  These test cross-experiments became hallmarks in the search for sex-linked traits .
Application in model organisms
Test crosses have a wide variety of applications. Common animal organisms, called model organisms , where test crosses are often used, include Caenorhabditis elegans and Drosophila melanogaster . The basic procedures for performing test hybridization in these organisms are given below:
To perform a test cross with C. elegans, place worms with a known recessive genotype on an agar plate with worms of an unknown genotype. Give the male and hermaphrodite time to mate and produce offspring. Using the microscope, the ratio of recessive versus dominant phenotype will elucidate the genotype of the dominant parent.
To perform a test cross with D. melanogaster, select a trait with a known dominant and recessive phenotype. The red color of the eyes predominates and the white color is dominant. Obtain white-eyed virgin females, red-eyed young males and put them in the same tube. Once the offspring begin to appear as larvae, remove the parental lines and observe the phenotype of the adult offspring.
There are several limitations to testing the cross. This can be a time-consuming process as some organisms need to grow over a long period of time in each generation to show the required phenotype.  The statistics also require reliable data on large numbers of offspring.  Test crosses are only useful when dominance is complete. Incomplete dominance occurs when the dominant allele and the recessive allele together form a mixture of the two phenotypes in the offspring. Variable expressivity occurs when a single allele produces a variety of phenotypes that are not even accounted for in a test cross.
As more advanced techniques emerge for determining genotypes, the test cross is becoming less prevalent in genetics. Genetic testing and genome mapping are modern advances that allow more efficient and detailed information to be determined about one’s genotype.  Test crosses, however, are still used today and have formed an excellent basis for the development of more sophisticated techniques.