What is the purpose of performing a test cross?
Test cross is done to determine the genotype of a plant regarding a particular trait.
Let's take a tall pea plant of unknown genotype.
In pea plant tallness is dominant over dwarfness.
We do not know whether given plant is homozygous tall (TT) or heterozygous tall (Tt ).
The genotype of the plant can be determined by performing a test cross.
Test cross is crossing a plant of unknown genotype with the recessive homozygous plant regarding that trait.
The gametes of the recessive homozygous plant will not have any influence on the phenotype of the offsprings as all the gametes will carry recessive allele.
We can determine the genotype of the given plant by observing the phenotype of the offsprings produced in test cross.
The phenotypes of the offsprings are observed.
The given plant is homozygous tall (TT), If all the offsprings are tall, heterozygous tall (Tt) if both tall and dwarf offsprings are produced. This is because the phenotypes produced depend upon the gametes produced by the plant of unknown genotype. The gametes of dwarf plant do not contribute to phenotype as all carry recessive allele 't'.