Inheritance of skin colour in human is controlled by three pairs of genes, $\mathrm{A}, \mathrm{B}$ and $\mathrm{C}$ which is polygenic inheritance. Codominance is shown by Blood group $\mathrm{AB}$. Chromosomal aberration and point mutation change the genetic sequence of human genome.
Related Theory
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). 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. Some examples of polygenic inheritance are: human skin and eye colour; height, weight and intelligence in people; and kernel colour of wheat.