They are the one-in-a-million babies. Kaydon Richardson is black. His twin brother Layton is white.

The boys were born in Middlesbrough, England, on July 23 - four weeks prematurely - with Layton weighing 2.83kg (6lb 4oz) and Kaydon, who arrived 20 minutes later, 2.97kg (6lb 9oz).

Their English-Nigerian mother, Kerry Richardson, aged 27, from Middlesbrough, says that at their birth the twins were "practically the same colour". "But over the last few months Layton has got lighter and blonder, like his dad [who is white], and Kaydon has gone darker like me."

Scientists believe their conception is a one-in-a-million chance. Clinical geneticist Dr Stephen Withers said the likelihood of a mixed-race woman having eggs predominantly for one skin colour was rare enough, let alone releasing two of them simultaneously and producing twins.