Practice question

A dihybrid test cross intercrosses double heterozygote AaBb with homozygous recessive tester aabb, directly exposing gamete constitution. When two genes assort independently, heterozygote meiosis generates four gamete types AB, Ab, aB, ab in equal 25% frequency due to random chromosome alignment at metaphase I and independent segregation. Tester parent contributes only ab gametes, so progeny genotypes AaBb, Aabb, aaBb, aabb appear equally, producing phenotypic ratio 1:1:1:1. Any significant deviation toward excess parental types indicates linkage and permits calculation of recombination frequency for constructing genetic linkage maps.