The Secret Life of Twins

Two hundred and fifty pairs of twins gather at St Thomas' Hospital to celebrate their contribution to science, while identical twin doctors Chris and Xand Van Tulleken set out to discover if their shared genes mean they will forever be identical. Meet twins raised apart as they're finally reunited, twins who suffer identical illnesses at the same time, and find out how identical twins hold the key to understanding what makes us all who we are.

The second programme in this two-part series focuses on how the differences between identical twins can help science understand what makes us all who we are. Through these personal stories we look at aspects of health and lifestyle that affect us all, obesity, ageing and the fight against killer diseases including cancer. We meet identical twins Mark and John who are helping scientists to explore the possible genetic basis of sexuality, they may be genetic clones but John is gay and Mark straight - they want to understand what makes them different.