You may have seen pictures or heard stories where “twins” look completely different. One has blonde hair, while the other is a brunette. One is very tall, while the other is on the shorter side. One is black, while the other is white — wait what? How is this possible?

“Twins who look so different are very striking to the general public, but to a geneticist, it’s not really that weird,” says Janet Boughman, executive vice president of the American Society of Human Genetics.

In a case where both parents are bi-racial, a type of genetic mixing can be created with their dark-skinned and light-skinned genes. One twin can inherit more of the dark-skinned genes, while the other can inherit more of the light-skinned genes. The same idea can be true with one Caucasian parent and one African American parent.

Although black and white twins are very rare, there is a chance that they can become more common as the number of interracial couples grow.