The equilibrium is the direct consequence of the segregation of alleles at meiosis in heterozygotes.
Numerically, the equilibrium shows that irrespective of the particular mixture of genotypes in the parental generation, the genotypic distribution after one round of mating is completely specified by the allelic frequency p.
One consequence of the Hardy-Weinberg proportions is that rare alleles are virtually never in homozygous condition. An allele with a frequency of 0.001 occurs in homomygotes at a frequency of only one in a million; most copies of rare alleles are found in heterozygotes. In general, since two copies of an allele are in homozygotes but only one copy of that allele is in each heterozygote, the relative frequency of the allele in the heterozygotes(as opposed to homozygotes) is:
Which for q=0.001 is a ratio 999:1. Thus, the frequency of heterozygote carriers of rare genes that are deleterious in a homozygous condition is much greater than the frequency of the affected homomygotes.
In our derivation of the equilibrium, we assumed the allelic frequency p is the same in sperm and eggs.