Type I diabetes is autoimmune. The immune system attacks the cells in the pancreas which produce insulin. The body stops producing insulin, with dramatic results. Treatment is a combination of insulin by injection (in all cases) and diet. There is a tendency to run in families, but specific genes have not been identified. Formerly called juvenile diabetes, it can develop at any age.
I am originally from North Europe and I have HLA DQ2.5 & DQ8. They are both "celiac" genes. I've already got celiac . Also I know that this combination of genes puts me at a high risk for juvenile diabeties. Doesn't make me happy. Anyway, there IS a gene, associated with diabeties - DQ8:
"...In Europe, DQ8 is associated with juvenile diabetes and coeliac disease. The highest risk factor for type 1 diabetes is the HLA DQ8/DQ2.5 phenotype. In parts of eastern Scandinavia both DQ2.5 and DQ8 are high increases frequencies of late onset Type I and ambiguous Type I/II diabetes..." (from http://en.wikipedia.org/wiki/HLA-DQ8)