Genotype

A genotype is the genetic constitution of an organism — the particular alleles present at one locus or, more broadly, across the whole genome. Its counterpart is the phenotype, the set of observable characteristics that result from the genotype interacting with the environment. Two plants with the same genotype can differ in height if one receives more light, and two organisms with different genotypes can share the same phenotype.

For a single gene with two alleles, an organism is homozygous if both copies are the same (AA or aa) and heterozygous if they differ (Aa). With simple dominance, AA and Aa produce the same dominant phenotype while only aa shows the recessive one — which is why a phenotype alone often cannot reveal a genotype. A monohybrid cross between two heterozygotes, Aa × Aa, produces the classic 1:2:1 genotypic ratio and 3:1 phenotypic ratio, worked out with a Punnett square.

Not every gene follows simple dominance. Incomplete dominance produces a blended intermediate phenotype in heterozygotes, codominance expresses both alleles fully as in the AB blood group, and many traits are polygenic, shaped by numerous loci plus environment. In populations, genotype frequencies under the Hardy–Weinberg model follow p² + 2pq + q² = 1, which lets you estimate carrier frequency from disease prevalence.

Genotype is foundational content across several exams. The MCAT tests Mendelian concepts and genotype–phenotype relationships in the biology and biochemistry section, USMLE Step 1 applies genotype reasoning in genetics fundamentals and population genetics including Hardy–Weinberg calculations, and the ACT science section expects the term as prior knowledge in genetics-based passages.