F1 generation

The F1 generation is the direct children of the parental (P) cross — the first batch of offspring in a breeding experiment. The 'F' stands for filial, from the Latin for son or daughter, and the '1' marks it as the first such generation.

When the P generation is two true-breeding parents that differ in a trait, every F1 individual is heterozygous, carrying one allele from each parent. In a simple dominant-recessive case this makes the whole F1 generation uniform, all displaying the dominant trait, even though each one quietly carries the recessive allele.

That uniform, hybrid F1 is what gives the next generation its interest. Because the F1 is heterozygous, letting F1 individuals interbreed lets the masked recessive allele reappear in the F2 generation in predictable ratios — the step where Mendel's hidden factors revealed themselves.