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It is a method to determine the genotype of an organism.

A test cross is a cross between an individual with an unknown genotype with a homozygous recessive genotype. It is a method to determine the genotype of an organism. The genetic makeup of an individual is known as its genotype. The genotype exhibits the alleles or the gene pattern carried by an organism. Earlier, it was used as an experimental mating test which was used to determine the alleles present in the genotype. Therefore, a test cross can be used to determine if a dominant phenotype is heterozygous or homozygous in nature for a particular allele.

Humans have diploid cells, each  genetic  spot has two alleles, wherein each allele is derived from each of the parents. Not necessarily, equal phenotypes or visible outcomes are produced by different alleles all the time. In a heterozygous entity, one allele might be dominant and conceal the effect of the other recessive allele which carries two different alleles at a particular position. In an entity, recessive alleles are able to express their phenotypes only if it carries two identical copies of the recessive alleles, implying homozygous for the recessive allele. This indicates that the dominant phenotype in the genotype of an entity can either be heterozygous or homozygous for the dominant allele. Hence it is not possible to detect the genotype of an entity with a dominant trait via visual examination of its phenotype.

A test cross can be performed to determine if an entity exhibits a dominant trait with homozygous or heterozygous for a specific allele. The entity being tested is crossed with a homozygous entity for the recessive trait and the progeny of this test cross is examined. If the outcome of the test cross is a recessive progeny, then it is deduced that the parent is heterozygous for the allele being tested. If the outcome of the test cross is a dominant progeny phenotypically, then the parent is a homozygous dominant for the allele being tested.

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