Starting next week BBC One begins a short series of programmes in which three celebrities go on a mission to discover the source of their defining trait, talent or characteristic. What is it that makes them tick? That gives them what they need to be who they are? Combining science, psychology, brain scans and genetics, they attempt to answer the question we've all asked at one time or another: how have we been shaped by nature and nurture?

In the first programme, John Barrowman embarks on a journey of self-discovery to find out the truth about his sexuality. John is a successful actor, musical performer, dancer, singer and TV presenter. For as long as he has been aware of his sexuality, John has been convinced that he was born gay and firmly believes that homosexuality is not a choice. But did nature really mould him or did nurture have a part to play?

John undertakes a series of tests including a DNA examination that is compared with his straight brother's, and he takes to the streets of LA to try out a theory that the origin of homosexuality is linked to events in the womb that may affect the foetus.

Throughout the programme, as well as picking up clues from psychologists and scientists, John also meets some celebrities and extraordinary people who help shed light on the origins of their traits. He's also beginning to realise that as a high-profile actor in shows such as Doctor Who and Torchwood, he has a role to play in helping young people come to terms with their sexuality. He has said recently that he hopes the programme will also be watched by the parents of gay youngsters.

In the second of the series, Olympic silver medallist and 110m hurdles world record holder Colin Jackson is tested, almost to destruction, in a bid to find the answers and discovers what it was that made him a great athlete.

The final instalment sees violin virtuoso Vanessa Mae embarking on a journey of self-discovery to find out what made her a child prodigy.

John Barrowman - The Making Of Me: BBC One, Thursday 24 July, 9pm