I thinks its 5% if the mother has T1 7% if the father, and if both it’s abt 12%. Not sure I’m correct but it’s definitely in this range. I’m also not sure about the general population, will have to look that up.
tralala, there’s a 50% chance only by identical twins. That means if one twin has T1 the other (identical) has a 50% chance to get it too.