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. Different alleles or gene variants code for differences in the melanin found within the skin. Within some groups are high frequencies of dark skin alleles, while others have high frequencies of light skin alleles, for example. The parents of such twins, who are typically both of mixed race, have a combination of alleles for light and dark skin in their genome.
Each sperm or egg cell possesses a random selection of genes from its mother or father. While not the most probable event, a sperm or egg may randomly acquire, for example, mostly alleles that confer light skin coloration or mostly alleles that confer dark skin coloration. In such cases, fraternal twins can differ from each other quite dramatically in terms of skin color or other physical characteristics.