Some of these mutations, likely given to one of the twins only, can be bold and hapless. In nearly three-quarters of cases of monozygotic twins having an autoimmune disease, only one twin is affected. This is called, “disease discordance,” and includes conditions like hemophilia, multiple sclerosis, and Duchenne muscular dystrophy. Rarely, some "identical" twins may even end up with differing sex chromosomes. Monozygotic twins, as they should be called, may appear identical at first glance, however, very complex dynamics are at play under the surface.
By Katherine Morone