I came across this interesting study that found that in identical twins. only 40% of the identical twins of a Type 1 diabetic will also become Type 1 diabetics. That makes T1D at least 60%
not genetic. I say
at least, because there will have been common environmental factors and upbringing for the twins (unless separated at birth, and even then there will be a common pregnancy).
Barnett AH, Eff C, Leslie RD, et al. Diabetes in identical twins. A study of 200 pairs. Diabetologia. 1981;20: 87–93.
http://www.ncbi.nlm.nih.gov/pubmed/7193616
Cited in
http://www.columbia.edu/itc/hs/medical/pathophys/immunology/readings/IsetCellDestructionReview.pdf
In addition to the variability in the genetics, there is the variability of heredity - not all genes from either parent are passed on. So depending on the number of genes involved, this further reduces it. Eg there was a single recessive gene for diabetes, only 25% of children would have the active gene (a pair of the recessive genes) even when both parents were carriers.