Supplementary Genes - In supplementary genes, the dominant allele of one gene produces a phenotypic effect. The dominant allele of the other gene does not produce any phenotypic effect on its own. But when it is present with the dominant allele of the first gene, it modifies the phenotypic effect produced by the first gene.

That is, the dominant allele of one gene is necessary for the development of the concerned phenotype, while that of the other gene modifies the phenotypic expression of the first gene. This interaction shows a ratio of 9:3:4.For example, the development of aleurone colour in maize is governed by two completely dominant genes Rand P.