The Principle of Segregation States That Each Individual Organism Possesses

The principle of segregation states that each individual organism possesses two alleles that can encode a characteristic. These alleles segregate when gametes are formed, and one allele goes into each gamete. The concept of dominance states that, when the two alleles of a genotype are different, only the trait encoded by one of them-the "dominant" allele-is observed.
-How did Mendel know that each of his pea plants carried two alleles encoding a characteristic?