phenotype ratio

The phenotype ratio counts how the offspring of a cross sort out by visible traits — by what they actually look like — rather than by their hidden genetic make-up. It is the ratio you could measure just by inspecting the offspring, without any genetic test.

The most famous example is the 3:1 ratio from a monohybrid cross of two heterozygotes: three offspring showing the dominant trait for every one showing the recessive trait. A dihybrid cross of two double-heterozygotes gives the equally celebrated 9:3:3:1. These ratios fall directly out of a Punnett square once each genotype is translated into its appearance.

Because several genotypes can share one appearance, the phenotype ratio is usually coarser than the genotype ratio behind it. The neat numbers also assume simple full dominance; incomplete dominance, codominance, lethal alleles, gene interactions, or environmental effects can reshape the ratio into something like 1:2:1 or 9:3:4, and an unexpected ratio is often the first clue to such complications.