If you have type 1 diabetes, your first-degree relatives (i.e. mother, father, brother, sister, and your children) are about ten times more likely than the general population to get type 1 diabetes. The frequency of type 1 diabetes in the general population is about half a percent (i.e., one in two hundred), so the risk in your first-degree relatives is about 10 1/2, or 5%. Fortunately, this is not particularly high.
Also, it is related to age. About 90% of cases of type 1 diabetes occur before age 35. Therefore, the parents of a person with type 1 diabetes are at considerably less, and ever decreasing, risk compared to his or her sib-lings and children. The risk of getting type 1 diabetes for an identical twin of a person with type 1 diabetes is 30–50%, indicating that environmental factors are very important, even though they are not well understood. In the case of type 2 diabetes, the risk is considerably higher.
On average, one out of three of the children of an individual with type 2 diabetes will develop the disease. Two out of three of the children will develop diabetes when both parents have the disease. The risk of getting type 2 diabetes for an identical twin of a person with type 2 diabetes is 75–90%, indicating that genetic (hereditary) factors are very important.