In the F2 generation, 3/4 of the plants exhibit the same
(dominant) trait as the F1 generation,
and 1/4 exhibit the contrasting trait that disappeared in the F1 generation.
To explain these results, Mendel proposed the existence of particulate unit factors (genes) for each characteristic (phenotype).
The two contrasting traits of the same gene are called alleles. An individual possessing two copies of the same allele is homozygous. An individual possessing different copies of an allele is heterozygous.
The F2 genotype ratio is 1:2:1; while the phenotype ratio is 3:1.