Medical Genetics The multifactorial model is thena (1) Several, but not an unlimited number oci are involved in the expression of the trait (2) There is no dominance or recessivity at each of these loci (3 The loci act in concert in an additive fashion, each adding or detracting a small amount from the phenotype (4)The environment interacts with the genotype to produce the final phenotypeMedical Genetics The multifactorial model is then: (1) Several, but not an unlimited number, loci are involved in the expression of the trait. (2) There is no dominance or recessivity at each of these loci. (3) The loci act in concert in an additive fashion, each adding or detracting a small amount from the phenotype. (4) The environment interacts with the genotype to produce the final phenotype