The Y chromosome. What makes a boy a boy? In this programme we put the Y chromosome under the microscope. We find out how it transforms a female embryo into a male one. We discover what it can tell us about the differences between men and women. We speak to a teenage boy who has not just one Y chromosome but two. And we meet the scientist who turned a female mouse male.

(Photo: X and Y chromosome on a black background. Credit: Shutterstock)