A test cross is performed to
Test cross is performed to determine whether individual expressing dominant phenotype possesses homozygous dominant TT or heterozygous Tt genotype, indistinguishable under complete dominance. Crossing unknown dominant to homozygous recessive tt forces tester to contribute only recessive allele, so offspring phenotype directly reveals gametes produced by tested individual. Appearance of recessive progeny indicates heterozygosity, absence suggests homozygosity. Purpose not increased variation or hybrid production, but assessment of genotype purity essential for breeding programmes and validating Mendelian predictions. Conceptual clarity supports solving numerical problems involving segregation ratios, recombination frequencies and probability calculations in crosses.
Ref: Hartl & Ruvolo, Genetics, 9th ed., Chapter 2: Test Cross Purpose