allele

Meaning: Alleles are variants of one gene that differ in DNA sequence, usually because of past mutations. A diploid organism inherits one allele from each parent. Identical alleles make it homozygous at that gene; different alleles make it heterozygous. A dominant allele shows its phenotype in a heterozygote, while a recessive allele shows only in a homozygote. Not every gene follows this pattern: incomplete dominance blends the phenotypes, codominance expresses both, and a population can hold more than two alleles of one gene (ABO blood type has IA, IB and i). The two alleles separate into different gametes during meiosis (Mendel's law of segregation).

Equation: p + q = 1, where p and q are the frequencies (unitless, between 0 and 1) of two alleles in a population. At Hardy-Weinberg equilibrium, p² + 2pq + q² = 1 gives the genotype frequencies.

On the MCAT: Expect Punnett squares, pedigrees, and Hardy-Weinberg problems. A typical problem gives the frequency of a recessive phenotype (q²), then asks you to take the square root to find q, subtract from 1 to find p, and calculate carriers as 2pq. Passages also ask how selection, drift, or gene flow change allele frequencies.

Common confusion: A gene is not an allele. The gene is the locus, and alleles are its versions. Dominant also does not mean common or beneficial. A dominant allele can be rare in a population, and a recessive allele can be very common.