Practice question

Identifying heterozygosity requires revealing masked recessive allele hidden by complete dominance. Crossing individual of unknown genotype to homozygous recessive tester aa allows both alleles from unknown parent to express phenomenologically. If unknown is AA, all progeny receive A and display dominant phenotype; if unknown is Aa, half progeny receive A, half a from that parent, producing 1 dominant :1 recessive ratio. Appearance of recessive class in progeny therefore proves heterozygosity. This test cross principle underlies breeding purity testing and pedigree verification in Mendelian genetics.