Nonetheless, there is one very simple situation for which analytical calculation is possible and helpful: that of the simplest disease model in which only a single SNP, correlated with the causal variant, is genotyped. For a design with the same number of cases and controls, under the disease model in which disease risk changes multiplicatively with the number of copies of the risk allele carried by an individual (this model is often referred to as the additive model because risk increases additively on the log scale), there is a known analytical relationship [13]: (1)where χ 2 is the chi-squared test statistic, the number of cases and controls, γ the effect size, p the allele frequency of the risk variant and γ 2 is the correlation between the marker and causal SNP.
