<<Disclaimer: Verify this information before applying it to your situation.>> Many responded that both twins have celiac, and of course some don't. "There have been studies done on this. There is a 70% concordance between identical twins. That is, if one twin is celiac there is a 70% chance that the other will be also. That's how we know that CD is primarily hereditary, but also involves an environmental trigger" Lila - ND