[knightni]

Hereditary, but not in a strong sense - to quote the american diabetes association:

> In general, if you are a man with type 1 diabetes, the odds of your child getting diabetes are 1 in 17. If you are a woman with type 1 diabetes and your child was born before you were 25, your child's risk is 1 in 25; if your child was born after you turned 25, your child's risk is 1 in 100.

Even if both partners are diabetic, the risk is only 1 in 10 to 1 in 4. This strongly implies that what is heritable is the vulnerability to getting diabetes, rather than getting diabetes itself. There is, presumably, some trigger that also needs to occur.