Polygenic (Quantitative) inheritance

 In human population, characters such as height, skin colour and intelligence show gradations (continuous variations) in expressions and not only two contrasting expressions. These characters are determined by two or more gene pairs, and they have additive or cumulative effect. These genes are called cumulative genes or polygenes or multiple factors. The ratio is 1:4:6:4:1 if there are two genes having additive effect (e.g. Wheat kernel colour) and in case three such genes (polygenes), the ratio is 1:6:15:20:15:6:1 (e.g. skin colour in human).