complete dominance

Meaning: In complete dominance, the dominant allele produces a phenotype that fully masks the recessive allele when both are present, so the heterozygote (Aa) looks like the homozygous dominant (AA). Typically the dominant allele encodes a functional product, so one working copy is enough, while the recessive allele often encodes a nonfunctional product. Only the homozygous recessive (aa) shows the recessive phenotype.

On the MCAT: Expect Punnett squares and pedigrees. Aa × Aa typically gives 3:1 phenotypes and 1:2:1 genotypes. A test cross (unknown × aa) shows the genotype: any recessive offspring means the dominant-phenotype parent is Aa. In pedigrees, two unaffected parents with an affected child suggest a recessive trait; two affected parents with an unaffected child suggest a dominant trait. Contrast it with incomplete dominance (intermediate phenotype) and codominance (both alleles expressed, as in AB blood type).

Common confusion: Students treat dominant as stronger, better or more common. Dominance describes the heterozygote's phenotype, not how common an allele is. A dominant allele can be rare and a recessive allele common. Allele frequency depends on forces such as selection, drift, mutation and gene flow (departures from Hardy-Weinberg equilibrium), not on dominance.