Polygenic inheritance is defined as characters which are determined by two or more gene pairs, and they have additive or cumulative effect. Classic example is the human skin colour which is controlled by three pairs of genes. When a dark skinned individual male and a white skin female have children, they will get alleles of both characters and are generally referred to as mulatto. All shades of colours can be expressed in the population like very dark, dark, fairly dark intermediate, fairly light, light and very light respectively in the ratio $1\mathrm{ }:\mathrm{ }6\mathrm{ }:\mathrm{ }15\mathrm{ }:\mathrm{ }20\mathrm{ }:\mathrm{ }15\mathrm{ }:\mathrm{ }6\mathrm{ }:\mathrm{ }1$