This week, the Internet has been fascinated by a set of English fraternal twins Lucy and Maria Aylmer. But there’s something special about them: Lucy has pale skin and red hair, while Maria has brown skin and brown curly hair.

In other words, one twin looks white, and the other twin looks black.

Their parents have a mixed racial background: their mother is half Jamaican, and their father is white.

Occasionally, stories like theirs pop up every now and again. In 2009, another British mixed-race couple produced not one, but two, sets of identical twins who each looked very racially different from their sibling.

But how common is this?

Unfortunately, there are no statistics that track this. From “The Associated Press:”

The phenomenon is so uncommon that there are no statistics to illustrate its probability, although it is thought likely to become more common because of the growing number of mixed-race couples.

To give you an idea on exactly how uncommon this is (using numbers!), Dr. Sarah Jarvis of Britain’s Royal College of General Pracitioners, said in 2009 (though it still applies today):

“Even non-identical twins aren’t that common. Non-identical twins from mixed parents, of different races, less common still. To have two eggs fertilized and come out different colors, less common still. So, to have it happen twice must be one in millions.”

But that’s just a guess, though the BBC reported chances closer to 1 in 500 in 2011. We won’t know until we actually start tracking the numbers.