Two brothers, both alike in heritage, but as different as black and white. Ebony and ivory. Night and day. You get it. If one thing's clear about 18-year-old British twins James and Daniel Kelly, it's that they never get confused for one another. James and Daniel are the human version of a black and white cookie. Born to Alyson and Errol Kelly, an interracial couple, they display the unusual characteristic of being a pair of one dark-skinned and one-light skinned twins:So how did this genetic anomaly occur?That the cause is the father's heritage.Jamaican by background, holds the genetic key to skin color variations among offspring. "It wouldn't really be possible for a black African father and a white mother to have a white child, because the African would carry only black skin gene variants in his DNA, so wouldn't have any European DNA, with white skin variants, to pass on,""However, people of Caribbean descent are often likely to carry European DNA. Which, if you can remember back to your high school biology unit on genetics, is enough to create a striking difference."The Caribbean father will have less European DNA than African DNA, so it's more likely he'll pass on African DNA - but rarely, and I've worked it out to be around one in 500 sets of twins where there's a couple of this genetic mix, the father will pass on a lot of European DNA to one child and mostly African DNA to the other. The result will be one white child and one black."James and Daniel's differences are, of course, more than just skin-deep. James, the darker skinned twin, is gay, outgoing, and excels at academics. Daniel, on the other hand, is straight, introverted, and has little proclivity toward school.