As per Mendel's Law of Segregation/Law of purity of gametes "alleles segregate randomly into gametes: When gametes are formed, each allele of one parent segregates randomly into the gametes, such that half of the parent's gametes carry each allele."

Monohybrid cross is defined as "a genetic mix between two individuals who have homozygous genotypes, or genotypes that have completely dominant or completely recessive alleles, which result in opposite phenotypes for a certain genetic trait". In this cross the different alleles under consideration have one genetic locus and Mendel gave the law of purity of gametes or law of segregation based on his study in monohybrid cross.