A test cross of a dihybrid heterozygote results in which phenotypic ratio?
Testing double heterozygote AaBb with homozygous recessive aabb directly exposes its gametic output because tester contributes only ab alleles. Under independent assortment, heterozygote meiosis yields four gamete types AB, Ab, aB, ab equally at 25% each, generating progeny genotypes AaBb, Aabb, aaBb, aabb in equal 1:1:1:1 phenotypic ratio. This equal distribution distinguishes Mendelian independent assortment from linkage where parental allele combinations exceed recombinants. Consequently dihybrid test cross serves both as verification of independent assortment and as quantitative assay for recombination frequency and genetic distance.
Ref: Hartl & Jones, Genetics: Analysis of Genes and Genomes, 8th ed., Chapter 4: Dihybrid Test Cross 1:1:1:1