F2 generation

The F2 generation is the grandchildren of the original cross — the offspring produced when F1 individuals are bred together (or self-fertilised, as Mendel's peas often were). It is the generation where the interesting patterns of Mendelian inheritance finally surface.

Because every F1 parent is heterozygous, the F2 mixes alleles freely, and traits that vanished in the uniform F1 come back. For a single gene with simple dominance, the F2 shows the classic 3:1 phenotype ratio — three offspring with the dominant trait for each one with the recessive trait — underlain by a 1:2:1 genotype ratio.

These F2 ratios were the crucial evidence for Mendel's laws: the reappearance of the recessive trait demonstrated that hereditary factors stay intact rather than blending, and the steady numbers demonstrated their orderly segregation. The ratio expected in the F2 (3:1, 9:3:3:1, or some variant) is a fingerprint of the underlying genetics, so a measured F2 is one of the most informative steps in a classical cross.