The Hardy-Weinberg equilibrium law states that when a population is in equilibrium, the genotypic frequencies will be in the proportion p2 : 2pq: q2 . In a large random mating hypothetical population where the frequencies of alleles A1 and A2 are respectively is p and q, each genotype passes on both alleles with equal frequency over generations in the absence of evolutionary forces (Mutation, migration, and selection). In this paper, the Hardy-Weinberg equilibrium law is derived and extended to the third generation, and the corresponding proportion of frequencies is derived with all mating patterns. The mating frequency matrix is also given. Further, the law is generalized for multiple alleles and generations using binomial expans...