monohybrid cross

A monohybrid cross is the simplest controlled experiment in genetics: you breed two individuals while watching just one trait, and ignore everything else. By keeping attention on a single gene, the underlying pattern of inheritance becomes easy to see.

The classic version crosses two true-breeding parents that differ in one trait, then lets their hybrid offspring breed among themselves. Mendel did exactly this with peas — for instance tall versus short. Every plant in the first generation looked tall, but in the next generation tall and short reappeared in a roughly 3:1 ratio, revealing the hidden recessive allele.

That 3:1 phenotype ratio (underlain by a 1:2:1 genotype ratio) is the signature of a single gene with one dominant and one recessive allele. Real traits that depend on several genes or on the environment will not give such a clean ratio, so a deviation is itself informative — a hint that the simple one-gene story is incomplete.