Medical Genetics 3. THE MULTIFACTORIAL MODEL As more loci are included this binomial distribution quickly approaches the gaussian distribution or the bell shaped normal curve, observed with human quantitative traits. Three loci, each with three alleles, are enough to produce population frequencies indistinguishable from a normal curveMedical Genetics 3. THE MULTIFACTORIAL MODEL As more loci are included, this binomial distribution quickly approaches the Gaussian distribution, or the bell￾shaped normal curve, observed with human quantitative traits. Three loci, each with three alleles, are enough to produce population frequencies indistinguishable from a normal curve