CBSE Class 12-science Answered

Test cross is a cross between an organism with unknown genotype and a recessive parent. It is used to determine whether an individual is homozygous or heterozygous for a trait.

Example:

• Suppose you have a violet and white flower and violet color (P) is dominant to white (p). The white flower must be homozygous for the recessive allele, but the genotype of the violet flower is unknown. It could be either PP or Pp.
• A testcross will determine the organism's genotype. The unknown genotype can be determined by observing the phenotypes of the resulting offspring.
• If crossing the unknown dominant phenotype (PP or Pp genotype) individual with the recessive phenotype individual produces only dominant phenotypes (no recessive), then the unknown individual is homozygous dominant.
• If any recessive phenotypic individuals result from the cross, then the unknown individual must carry the recessive allele, and have the heterozygous genotype.