The ratio of dominant to recessive traits in Mendel’s F2 generation was 3:1. This is the classic phenotypic ratio produced when two heterozygous F1 individuals are crossed for a trait showing complete dominance.
Mendel began with true-breeding pea plants that differed in one characteristic, such as purple versus white flowers. Their F1 offspring all displayed the dominant trait. When those hybrids self-fertilized, the recessive trait reappeared in the F2 generation, revealing that it had not vanished but had been masked in the hybrids.
At the genetic level, the expected F2 genotype ratio is 1:2:1: one homozygous dominant offspring, two heterozygous offspring, and one homozygous recessive offspring. Because both homozygous dominant and heterozygous plants show the dominant phenotype, their visible traits combine to produce the 3:1 ratio.
The other listed ratios describe different situations. The 1:2:1 ratio is genotypic for a standard monohybrid cross, while 9:3:3:1 is associated with an idealized dihybrid phenotypic cross. The 3:1 result is a probability expectation, so small real samples may not match it exactly.