When germ cells form, the homologous chromosomes are randomly shared into two daughter cells. In this case, each germ cell and the embryonic stem cell initiating the development has a different combination of chromosomes inherited from the parents. This creates genetic differences in children of the same parents.

Siblings may differ very much in terms of their inherited traits, such as hair quality, eye colour. Similarly, siblings can also differ from each other in terms of traits that are also influenced by external factors, such as height or intelligence. The random distribution of alleles or gene forms also results in "uneven" heritability of inherited diseases between individuals.