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From a biological point of view, the differences in these fraternal or dizygotic twins from two biracial parents are not surprising. In humans, a relatively small number of genes are thought to be responsible for human skin color. There are differences in these genes called alleles. These alleles or genetic differences cause the differences in human skin color. Different races or ethnic groups tend to have different groups of alleles that create different mixtures of skin color: some ethnic groups have groups of dark skin alleles, others have light skin alleles, and other ethnic groups tend to have some mixture of both. The parents of these twins, who are both biracial, have a mixture of alleles for light and dark skin in their genome. Each sperm or egg cell will randomly select a subset of these genetic differences. While not the most probable event, a sperm or egg may randomly acquire only the genetic differences (alleles) that confer light skin color or only the genetic differences that confer dark skin color. In this case, there is a certain low probability of having non-identical twins where one twin was the product of a sperm and one egg both with primarily "lighter skin" alleles and the other twin was the result of a sperm and egg both with primarily "darker skin" alleles.
- Mixed Twins
- Daily Mail, February 21, 2006: "Black and White Twins"
- ABC's Good Morning America: "One Twin's White, The Other's Black"
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