Reviewed by: Mary Anne Clark, PhD
Polygenic inheritance refers to the kind of inheritance in which the trait is produced from the cumulative effects of many genes in contrast to monogenic inheritance in which the trait results from the expression of one gene (or one gene pair). In monogenic inheritance, the expression may be predicted according to a phenotypic ratio that follows Mendelian inheritance. Polygenic inheritance is a non-Mendelian form since it is controlled by multiple genes at different loci on different chromosomes expressed together in the same trait.
For example, if one pair of genes controls color, and red is dominant to white, then when you cross two heterozygotes (Aa), red and white progeny will appear in the ratio of 3:1. However if two pairs of genes control color and the dominant allele at both loci must be expressed to get red flowers, then crossing two heterozygotes (Aa Bb) will give you red and white flowers in a ratio of 9:7. This is a modification of the typical dihybrid Mendelian ratio of 9:3:3:1, in which three of the progeny groups all have the same phenotype.
Polygenic inheritance is also involved in quantitative traits, in which multiple gene loci each contribute in a similar way to the phenotype so that the total number of contributing alleles determines the phenotype. In humans, height, weight, and skin color are examples of quantitative traits. For instance, the height of an adult human is determined by not just a single gene but by more than 400 genes apart from the other non-genetic factors such as environment and nutrition. In quantitative traits, the Mendelian ratios are replaced by a normal distribution curve, with the two ends of the curve defined by the two extremes possible for the phenotype.