AP Syllabus focus:
‘Test crosses involve breeding an individual with a homozygous recessive partner to determine whether a dominant phenotype is homozygous or heterozygous.’
Test crosses are a classic genetics tool for inferring an organism’s genotype from its phenotype. They rely on predictable inheritance patterns when a dominant-phenotype individual is crossed with a recessive-phenotype partner.
Core idea and when to use it
A test cross is used when an individual shows a dominant phenotype but could have one of two genotypes. By choosing a mate with a known genotype, the offspring reveal which alleles the unknown parent contributes.
Practice Questions
FAQ
There is no fixed number; confidence rises with sample size.
In practice, aim for as many offspring as feasible, because a small brood can miss recessive phenotypes by chance even if the parent is heterozygous.
Treat “all dominant” as evidence, not absolute proof, of homozygous dominance.
Repeat the cross or increase offspring number to reduce the likelihood that chance alone hid recessive outcomes.
Yes, but scoring must be standardised.
Define clear phenotype categories before starting
Use blind scoring if possible
Consider quantitative measurements if the trait permits
You must prevent selfing to keep the tester parent as the only pollen/parental source.
Common approaches include removing anthers, bagging flowers, or using male-sterile lines, depending on the species.
A homozygous recessive tester contributes only recessive alleles, so offspring directly reveal which allele came from the unknown parent.
A heterozygous tester introduces dominant alleles that can mask what the unknown parent contributed, weakening the inference.
