An organism showing a dominant phenotype could be homozygous dominant (TT) or heterozygous (Tt), because both look the same. A test cross is used to find out which.In a test cross, the organism with the dominant phenotype is crossed with an organism that is homozygous recessive (tt) for the same gene. The homozygous recessive parent can only make t gametes, so the phenotype of each offspring reveals which allele it received from the unknown parent.If the unknown parent is TT:
TT × tt → gametes T and t → all offspring Tt
All offspring show the dominant phenotype.If the unknown parent is Tt:
Tt × tt → gametes T, t and t → offspring 1 Tt : 1 tt
About half the offspring show the dominant phenotype and half the recessive phenotype (a 1 : 1 ratio).So the appearance of even one offspring with the recessive phenotype shows that the unknown parent must be heterozygous. If all offspring are dominant, the parent is probably homozygous dominant. The more offspring produced, the more confident the conclusion, because with only a few offspring a heterozygous parent could produce all dominant offspring by chance.Worked example: a black guinea pig (B dominant to white b) crossed with a white guinea pig produces 5 black and 4 white young. The white young are bb, so they received b from the black parent. The black parent is therefore Bb.Test crosses are widely used by plant and animal breeders to check that stock is pure-breeding.
TT × tt → gametes T and t → all offspring Tt
All offspring show the dominant phenotype.If the unknown parent is Tt:
Tt × tt → gametes T, t and t → offspring 1 Tt : 1 tt
About half the offspring show the dominant phenotype and half the recessive phenotype (a 1 : 1 ratio).So the appearance of even one offspring with the recessive phenotype shows that the unknown parent must be heterozygous. If all offspring are dominant, the parent is probably homozygous dominant. The more offspring produced, the more confident the conclusion, because with only a few offspring a heterozygous parent could produce all dominant offspring by chance.Worked example: a black guinea pig (B dominant to white b) crossed with a white guinea pig produces 5 black and 4 white young. The white young are bb, so they received b from the black parent. The black parent is therefore Bb.Test crosses are widely used by plant and animal breeders to check that stock is pure-breeding.