When crossing pea plants, Mendel noticed that one of the parental traits would sometimes disappear in the first generation of offspring, called the F₁ generation, and could reappear in the next generation (F₂). He concluded that one of the traits must be dominant over the other, thereby causing masking of one trait in the F₁ generation. When he crossed the F₁ plants, he found that 75% of the offspring in the F₂ generation had the dominant phenotype, while 25% had the recessive phenotype.

Mendel’s model to explain this result had four parts. First, alternative versions of genes, called alleles, account for differences in traits. Second, an organism inherits two copies of each gene, one from each parent. Third, the presence of a dominant allele masks the recessive allele. Fourth, the two alleles for a trait are separated during gamete formation. This last part of the model is called the Law of Segregation. If a parent has two different alleles, or is heterozygous, these alleles will be equally and randomly separated during gamete formation. Scientists now understand that the separation of chromosomes during meiosis accounts for the segregation of parental alleles.

Let us think about the cross between a purple flowering plant (genotype PP) and a white flowering plant (pp). The resulting F₁ generation has purple flowers (Pp). This phenotype is accounted for by part (3) of Mendel’s model, as the dominant purple allele masks the recessive white allele. However, when the F₁ plants are crossed (Pp x Pp), the offspring can be either purple or white with a ratio of 3 purple to 1 white. The corresponding genotypic ratio is 1 PP : 2 Pp : 1 pp. This result is supported by Mendel’s Law of Segregation, as each plant received one allele from each parent.