For conducting GWAS it is critical that the sample is sufficiently large to provide enough statistical power to reduce Type I error and detect an association. The power of GWAS to detect an association is dependent both on the allele frequency of the disease variant as well as its effect size, expressed as a genotype relative risk (GRR). GRR is defined as the probability of a person with a specific disease variant to have a disease compared to the probability of a person without that gene variant. Larger sample sizes are required to detect alleles of low frequency or of small effect.
