Test Cross

A test cross is a technique used to determine the genotype of an organism expressing a dominant trait, such as purple flower color in pea plants.

Because a dominant allele is expressed over a recessive allele, organisms with the homozygous dominant genotype, containing two dominant alleles, appear the same as organisms with the heterozygous genotype, containing one dominant allele and one recessive allele.

A purple-flowered pea plant of unknown genotype is crossed with a white-flowered pea plant having a homozygous recessive genotype.

Because the genotype of the white-flowered parent is known, the offspring phenotypes reveal whether the unknown parent is homozygous dominant or heterozygous.

In one scenario, if all offspring from the test cross display the dominant phenotype, the unknown parent must be homozygous dominant for purple flowers.

All offspring are heterozygous and receive one recessive allele from the recessive parent and one dominant allele from the other parent.

In another scenario, if the offspring include equal numbers of dominant and recessive phenotypes, the unknown parent must be heterozygous.

All of the offspring still receive their recessive allele from the recessive parent.

Half of the offspring receive a dominant allele from the heterozygous parent and exhibit the dominant phenotype, while the other half receive a recessive allele and exhibit the recessive phenotype.