The previous slides presented risk information that pertain to Caucasian, African American and Asian populations in general. However, in families where there is already one person with T1D, the risk to unaffected relatives is much higher than that for the general population. For example, in Caucasian families, siblings of an individual with T1D are about 15 times more likely to develop the disease then a person from the general population (i.e., without a family history of the disease).

Since siblings share half their genes with each other, researchers often look at whether the HLA haplotypes carried by an unaffected sibling are the same as those carried by the affected individual. Unaffected siblings can share two, one or zero HLA haplotypes with their affected brother or sister. As can be seen, the T1D risk for a sibling who has the same two HLA haplotypes as their T1D sibling is quite high (about 25%). If they share one or zero HLA haplotypes with their affected sibling, their risk is much lower, about 8% and 1%, respectively. The fact that the risk for individuals who have no shared HLA haplotype is still increased compared to that for the general population suggests that genes other than those in the HLA region must also increase susceptibility for the disease.