dominant

Meaning: A dominant allele is expressed in the phenotype of a heterozygote, so Aa looks like AA. The recessive allele shows in the phenotype when the individual is homozygous recessive (aa). Often one working copy makes enough functional protein, so loss-of-function alleles tend to be recessive; some dominant alleles act by gain of function instead. Incomplete dominance (blended phenotype) and codominance (both alleles expressed) are exceptions to complete dominance.

Equation: p² + 2pq + q² = 1, where p is the frequency of the dominant allele A, q is the frequency of the recessive allele a, and p + q = 1 (all unitless). The dominant phenotype frequency is p² + 2pq.

On the MCAT: Aa × Aa gives 1 AA : 2 Aa : 1 aa, a 3:1 phenotype ratio. A test cross with aa reveals an unknown genotype: all dominant offspring suggest AA, while a 1:1 ratio indicates Aa. In pedigrees, two unaffected parents with an affected child point to a recessive trait. In Hardy-Weinberg problems, start from q², the frequency of the recessive phenotype, and take its square root to find q.

Common confusion: Dominant does not mean more common or more fit. A dominant allele can be rare, and a recessive allele can be frequent in a population. Dominance describes only which allele is expressed in the heterozygote.