dihybrid test crosses

Meaning: A dihybrid test cross mates an individual showing the dominant phenotype for two traits, such as AaBb, with a homozygous recessive (aabb). The recessive parent contributes ab gametes that hide nothing, so each offspring's phenotype reveals which gamete the tested parent supplied. The offspring ratio therefore shows the tested parent's genotype and gamete frequencies.

Equation: Recombination frequency (%) = (recombinant offspring ÷ total offspring) × 100, where offspring are counts (unitless); 1% equals 1 map unit, or 1 centimorgan.

On the MCAT: If the genes assort independently, expect four phenotypes in about a 1:1:1:1 ratio. If most offspring are parental types and few are recombinants, the genes are linked on one chromosome; fewer recombinants mean the genes lie closer together. Passages give offspring counts and ask for linkage or map distance.

Common confusion: Do not mix this up with the dihybrid cross AaBb × AaBb, which gives about 9:3:3:1 for independent genes. In the test cross the partner is aabb, so the ratio reflects the gametes of one parent only. Recombination frequency reaches about 50% at most, which is the value for unlinked genes.