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. Show less