heterozygous

Meaning: A heterozygous individual has two different alleles at one locus, one on each homologous chromosome, such as Aa. It arises when gametes carrying different alleles fuse. With complete dominance, the dominant allele determines the phenotype and the recessive allele is masked. With incomplete dominance the phenotype is intermediate, and with codominance both alleles are fully expressed.

Equation: 2pq = frequency of heterozygotes in a Hardy-Weinberg population, where p is the frequency of the dominant allele and q is the frequency of the recessive allele (both unitless fractions, with p + q = 1).

On the MCAT: An Aa × Aa cross gives 1 AA : 2 Aa : 1 aa, so half the offspring are heterozygous and the phenotype ratio is 3:1 under complete dominance. In a test cross with aa, a 1:1 offspring ratio shows the unknown parent was heterozygous. Hardy-Weinberg questions give q² (affected individuals), so you find q, then p, then 2pq for carriers. In pedigrees, two unaffected parents with an affected child must both be heterozygous carriers for an autosomal recessive trait.

Common confusion: Heterozygous does not mean the person shows both traits. Under complete dominance only the dominant trait appears. Also, males have one X chromosome, so for X-linked genes they are hemizygous, not heterozygous.