Dr. Page interprets the results of an audience-participation experiment comparing testosterone levels in males and females of varying ages. He then explains how the Y chromosome is inherited from father to son in a near clonal fashion. He demonstrates that successive inversions and deletions during mammalian evolution have reduced the Y chromosome to its present form—small and sparsely populated with genes. In some men, a deletion in the Y chromosome can lead to infertility. Dr. Page describes how intracytoplasmic sperm injection can help these men father children.