dominance

Meaning: Dominance describes how two alleles of one gene interact in a heterozygote. In complete dominance, the heterozygote has the same phenotype as the dominant homozygote. This often happens because one functional allele makes enough protein for the normal phenotype, while the recessive allele is a loss-of-function version. Two other patterns also appear: incomplete dominance, where the heterozygote is intermediate (red × white flowers giving pink), and codominance, where both alleles are fully expressed (AB blood type).

On the MCAT: Expect Punnett squares and pedigrees. Aa × Aa gives genotypes 1 AA : 2 Aa : 1 aa and phenotypes 3 dominant : 1 recessive. In a test cross, an individual with the dominant phenotype is crossed with aa; any recessive offspring show that the individual is Aa. In a pedigree, two unaffected parents with an affected child suggest a recessive trait. In Hardy-Weinberg problems, the recessive phenotype frequency equals q², which is why you start from it.

Common confusion: Students assume a dominant allele is the more common or more beneficial one. Dominance describes only the phenotype of the heterozygote. Allele frequency is a separate matter: Huntington disease is dominant and rare, and many recessive alleles are common in a population. So p is not automatically larger than q.