Q.Define Test Cross.

A test cross reveals an unknown dominant-phenotype individual's genotype by crossing it with a homozygous recessive.

Because a dominant phenotype can be produced by either a homozygous dominant genotype or a heterozygous genotype, simply looking at an organism is not enough to know its exact genetic makeup. Mendel devised the test cross to resolve this: the individual of unknown genotype is crossed with a homozygous recessive individual for the trait being studied, since the recessive parent can only contribute recessive alleles and so cannot mask the alleles coming from the unknown parent. If the unknown parent is homozygous dominant, every offspring of the test cross will receive at least one dominant allele from it and will therefore all show the dominant phenotype. If, instead, the unknown parent is heterozygous, the test cross offspring will show a 1:1 ratio of dominant to recessive phenotypes, directly revealing the segregation of th …