The genetics of diabetes mellitus, including the South African perspective.

By and large, essential diabetes mellitus is thought to be 50% inherited and 50% environmental. In insulin-dependent diabetes mellitus (IDDM) there is a strong link with the HLA system with regard to the inheritance of 'susceptible' diabetic genes, especially the DR3 and DR4 alleles. In IDDM environmental factors act in a predisposed individual to initiate an immune response with resultant beta-cell damage and destruction. Non-insulin-dependent diabetes mellitus (NIDDM) has no clear HLA link, but has been shown in studies of twins to have a stronger genetic basis than IDDM. In NIDDM environmental factors (race, ethnicity, diet, obesity) have an important influence on the clinical expression of the disease and the severity of complications in a genetically predisposed individual. The non-insulin-dependent diabetes of the young (NIDDY) variant and the phenomenon of chlorpropamide-primed alcohol-induced flushing both underline the heterogeneity of NIDDM. Because of the heterogeneous nature and multifactorial inheritance pattern of diabetes mellitus, accurate genetic counselling is not possible as yet. However, data to date suggest that it is unwise to advise prospective parents not to procreate, since the overall risk of the development of clinical diabetes mellitus is extremely low.