Picture supplied by National Geographic via AP Photos.

London - Most onlookers assume these inseparable 11-year-old girls are friends rather than family. 

But after years of double-takes, their parents are used to patiently explaining that Millie and Marcia Biggs are not only sisters, but twins – despite one being black and one white.

Now National Geographic magazine has made them cover stars, illustrating why it says the world needs to "rethink everything we know about race".

Their parents, Amanda Wanklin and Michael Biggs, of Birmingham, explain that from a young age the girls had similar features – but while Marcia had light brown hair and fair skin like her English-born mother, Millie had black hair and brown skin like her father, who is of Jamaican descent. 

"We never worried about it, we just accepted it," Mr Biggs told the magazine.

Their mother, a carer, added: "When they were first born, I would be pushing them in the pram, and people would look at me and then look at my one daughter and then look at my other daughter.

"And then I'd get asked the question: 'Are they twins?' Yes. 'But one's white and one's black.' Yes. It's genes."

READ: For decades, our coverage was racist - National Geographic

Appearing on US television this week, Millie gave her own perspective: "I think it's nice because people can't tell you that you're … white and they can't tell you that you're black because you're not, you're like kind of both."

Marcia added: "I think it's better to be different from other people because you can just be yourself." Their mother described the girls as a "one-in-a-million" miracle. But it is far from rare for non-identical twins to look more like one parent than the other. For mixed-race couples, fraternal twins' traits depend on variables such as where the parents' ancestors are from, geneticist Alicia Martin told National Geographic.