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"The Nobel Prize-winning genetecist only discovered the truth about his own genes in his 50s, when he applied for a US green card and saw his full birth certificate for the first time. His sister fell pregnant as an unmarried 17-year-old, and Paul's grandmother brought him up as her own son. He still doesn't know who his father was.

"There was no tragedy for me," he says, but "it must have been really tragic for my mother."