Black and white twins

So you've probably already heard about these twins - in a case of 1 to 1 million odds, they were born last year as one white twin, one black twin. The parents each have a white mom and black dad and in this explaintion (long but interesting) it gives the genetic reasoning:

The odds against of a mixed race couple having twins of dramatically different colour are a million to one. Skin colour is believed to be determined by up to seven different genes working together.

If a woman is of mixed race, her eggs will usually contain a mixture of genes coding for both black and white skin. Similarly, a man of mixed race will have a variety of different genes in his sperm. When these eggs and sperm come together, they will create a baby of mixed race.

But, very occasionally, the egg or sperm might contain genes coding for one skin colour. If both the egg and sperm contain all white genes, the baby will be white. And if both contain just the versions necessary for black skin, the baby will be black.

For a mixed-race couple, the odds of either of these scenarios is around 100 to one. But both scenarios can occur at the same time if the woman conceives non-identical twins, another 100 to one chance. This involves two eggs being fertilised by two sperm at the same time, which also has odds of around 100 to one.

If a sperm containing all-white genes fuses with a similar egg and a sperm coding for purely black skin fuses with a similar egg, two babies of dramatically different colours will be born. The odds of this happening are 100 x 100 x 100 - a million to one.