When the boys spent a year in the same school, Sam did fine, but John struggled and had some noisy meltdowns.

When the boys spent a year in the same school, Sam did fine, but John struggled and had some noisy meltdowns. / Jodi Hilton for NPR

Sam and John Fetters, 19, are identical twins in very different places on 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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Today's episode was produced by Rachel Carlson. It was edited by Rebecca Ramirez. Jon Hamilton checked the facts. Phil Elfors and Gilly Moon were the audio engineers.

Tags: autism spectrum  Science of Siblings  identical twins  Series: The Science of Siblings