Three celebrities go on a mission to discover the source of their defining trait, talent or characteristic in the new science series for the BBC The Making Of Me.

With the help of scientific testing, the latest psychology techniques, brain science and genetics, they attempt to answer a question that affects everyone – how do nature and nurture shape us?

In the first programme (airing on Thursday 24 July) John Barrowman embarks on a journey of self-discovery to find out the truth about his sexuality and challenges scientists to explain why he's gay.

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.

With the help of friends, family, psychologists and geneticists - and conquering his claustrophobia to have a brain scan - he asks whether nature or nurture determine who we are.

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.

John Barrowman: The Making of Me
Thursday 24 July, 9pm, BBC ONE
www.bbc.co.uk/bbcone