genotype

Meaning: The genotype is the pair of alleles an individual has at a locus, or the set across several loci. Homozygous means two identical alleles (AA or aa); heterozygous means two different alleles (Aa). The phenotype is the trait you can observe, produced by the genotype together with the environment. With complete dominance, AA and Aa look alike. With incomplete dominance or codominance, the heterozygote has its own phenotype.

Equation: p² + 2pq + q² = 1, where p is the frequency of the dominant allele and q is the frequency of the recessive allele (both unitless, p + q = 1). Here p² is the frequency of genotype AA, 2pq of Aa, and q² of aa. It applies to a population in Hardy-Weinberg equilibrium.

On the MCAT: Passages give a pedigree or a cross, and you infer genotypes from the phenotypes of parents and offspring. For Aa × Aa, the genotypic ratio is 1 AA : 2 Aa : 1 aa, and the phenotypic ratio is 3 : 1 under complete dominance. A test cross with aa reveals an unknown genotype. Hardy-Weinberg questions often start from q², the recessive phenotype frequency.

Common confusion: Students mix up genotypic and phenotypic ratios. The same phenotype does not mean the same genotype, since AA and Aa can look identical. Genotype also does not fix phenotype alone, because environment and incomplete penetrance can alter it.