If one child in a family has diabetes mellitus (DM), then there is a risk of up to one in ten that his brother or sister may be similarly affected.
But we still do not know why diabetes occurs. At least four genes are known to be important in determining sensitivity to the development of damaged islet cells in the pancreas and subsequent diabetes.
Having said that, most of the susceptibility to DM is environmentally acquired rather than inherited in our genes. For example, we know that certain viruses such as rubella, mumps and Coxsackie virus all have the ability to damage the insulin-producing cells in the pancreas.