Each model makes different assumptions about the genetic effect in the data – assuming two alleles for a SNP, and , a dominant model (for ) assumes that having one or more copies of the allele increases risk compared to (i.e. or genotypes have higher risk). The recessive model (for ) assumes that two copies of the allele are required to alter risk, so individuals with the genotype are compared to individuals with and genotypes. The multiplicative model (for ) assumes that if there is 3× risk for having a single allele, there is a 9× risk for having two copies of the allele: in this case if the risk for is , the risk for is . The additive model (for ) assumes that there is a uniform, linear increase in risk for each copy of the allele, so if the risk is 3× for , there is a 6× risk for - in this case the risk for is and the risk for is . A common practice for GWAS is to examine additive models only, as the
