I don't know if this is as significant as fingerprints or not, but I have a coworker with an identical twin, and her sister has Crohn's disease, and my coworker

I don't know if this is as significant as fingerprints or not, but I have a coworker with an identical twin, and her sister has Crohn's disease, and my coworker does not. Chron's is apparently genetic, but there must be some environmental factors as well.