recessive

Meaning: A recessive allele is one whose trait is hidden when a dominant allele is also present, under complete dominance. The trait appears in homozygous recessive individuals (aa); heterozygotes (Aa) show the dominant phenotype and are carriers. Often the recessive allele encodes a nonfunctional protein, and one working copy is enough. For X-linked recessive traits, males have one X and show the trait with a single copy.

Equation: p² + 2pq + q² = 1, where p is the dominant allele frequency and q is the recessive allele frequency (both unitless, 0 to 1). q² is the frequency of homozygous recessive individuals in a population at Hardy-Weinberg equilibrium.

On the MCAT: Pedigrees and Punnett squares. Two unaffected parents with an affected child point to a recessive trait with both parents carriers. Aa × Aa gives 1/4 aa, and 2/3 of unaffected children are carriers. In Hardy-Weinberg questions, take the square root of the disease incidence to get q, then use 2pq for carriers.

Common confusion: Recessive does not mean rare or harmful. Dominance describes how an allele shows in a heterozygote, not how common it is in a population. Incomplete dominance and codominance are different: the heterozygote is intermediate or shows both traits.