genotype ratio

The genotype ratio is a tally of the different genetic make-ups among the offspring of a cross, written as a set of numbers like 1:2:1. It answers the question 'what allele combinations did the children inherit?' before we even ask what they look like.

The textbook example comes from crossing two heterozygotes (Aa x Aa). Their offspring fall into three genotypes — homozygous dominant, heterozygous, and homozygous recessive — in the ratio 1:2:1. A Punnett square makes this visible: of the four equally likely boxes, one is AA, two are Aa, and one is aa.

Genotype ratios are finer-grained than phenotype ratios, because two individuals with different genotypes can look identical. The classic 1:2:1 genotypes collapse to a 3:1 phenotype ratio whenever one allele is fully dominant. Like all Mendelian ratios, these are expected proportions over many offspring, not guarantees for any single family.