Practice question

Valid complementation test requires recessive loss-of-function alleles because dominant mutant phenotype expresses even with wild-type homologue present, preventing distinction between same-gene versus different-gene scenarios. Recessive mutants show phenotype only when both copies defective, so heterozygous genotype carrying one wild allele per locus can reveal whether functional copies exist for both loci. With dominance trans heterozygote always displays mutant trait regardless of locus identity yielding universal non-complementation and uninterpretable results. Therefore analysis restricted to recessive alleles where wild allele can supply normal function.