What is the principle of Hardy-Weinberg law?

Hi.  The Hardy-Weinberg theorem states that the allele and genotype frequencies remain constant from one generation to the next.  It explains how genetic variation is maintained in a population that is not evolving.  Such a population is in a state referred to as Hardy-Weinberg equilibrium.  The equation p^2 + 2pq + q^2 = 1 represents the GENOTYPE frequencies within a population.  P represents the frequency of one ALLELE, and q represents the frequency of another allele.  The genotype frequencies (and allele frequencies) always equal to 1.  The Hardy-Weinberg equation is depicted as follows:

p^2 = homozygous dominant (individual carries TWO alleles that code for a domiant trait).  In this case, both the male and female gametes provide a dominant allele.

2pq = heterozygous.  The (2) indicates that the heterozygous genotype can arise in two different ways.  The male gamete can provide the recessive allele, while the female gamete provides the dominant allele.  Or, the male gamete can provide the dominant allele, while the female gamete provides the recessive allele.

q^2 = homozygous recessive (individual carries TWO alleles that code for a recessive trait).  In this case, both the male and the female gametes provide the recessive allele.

I hope this helps.  I tried!