What is the crossing of an individual of unknown genotype with a homozygous recessive individual to determine the unknown genotype?

• A. test cross
• B. monohybrid cross
• C. back cross
• D. dihybrid cross

Answer: (A)

A tester cross is used to reveal an unknown genotype by crossing with a homozygous recessive individual. By observing the offspring, you can deduce what alleles the unknown parent carries. If every offspring shows the dominant trait, the unknown is homozygous for that trait. If about half show the dominant trait and half show the recessive trait, the unknown is heterozygous. This works because the homozygous recessive partner can only contribute recessive alleles, so any recessive phenotype in the offspring must come from the unknown parent. The other options don’t fit this purpose: a monohybrid cross involves just crossing for a single trait with two different alleles but isn’t specifically a tester cross; a back cross can involve crossing to a parental genotype but is not defined as the method used to determine the unknown genotype; a dihybrid cross looks at two genes simultaneously, not at identifying genotype for a single gene.