The Principle Of Segregation
Mendel used his observations to formulate his First Law, the Principle of Segregation. According to this principle, each gamete receives from a parent cell only one of the two alleles the parent cell carries for each trait, and the gamete has an equal chance of getting either allele. (Exceptions to the principle were described later by Thomas Hunt Morgan, an American geneticist.) When the two gametes unite during fertilization, the resulting cell contains two alleles, either identical or different, for each trait. These two alleles are referred to as the individual's genotype for the trait.
An alternative idea that other scientists during Mendel's time had was that two parental characteristics fused and blended into a single hybrid characteristic. Mendel's results showed this was not the case. His results showed, instead, that individuals inherit from their parents intact units that can leap through time. Mendel's discovery that inheritance had a particulate nature set the stage for modern advances in genetics.