>>23108601
You can't find them with all black twin siblings as well. What we discovered in dog and fox breeding in the last century was that pigmentation traits are something that happens more and more as you cross breed. Dogs have litters while humans generally have one child. As an example my cat recently gave birth to two orange white kittens, two all black, and one white. She was grey and white.
This can occur in humans too. Here are biracial twins to a White father and Jamaican Black mother.