Conditions for Hardy-Weinberg law to operate is

Conditions for Hardy-Weinberg law to operate are Population size should be large, Open pollination should occur, and Selection should not be carried out.
The Hardy-Weinberg principle, states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences.
Small population sizes can cause a random change in allele frequencies. This is due to a sampling effect and is called genetic drift.
Selection, in general, causes allele frequencies to change, often quite rapidly. While directional selection eventually leads to the loss of all alleles except the favoured one.