Sam and John Fetters, 19, are identical twins on different ends of the autism spectrum. Sam is a sophomore at Amherst College and runs marathons in his free time. John attends a school for people with special needs and loves to watch Sesame Street in his free time. Identical twins like Sam and John pose an important question for scientists: How can a disorder that is known to be highly genetic look so different in siblings who share the same genome?

Check out more of NPR's series on the Science of Siblings.

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