Theoretical considerations on HLA-associated illness-susceptibility genes in dermatitis herpetiformis and psoriasis.

The frequency of an HLA-associated illness-susceptibility (Is) gene can be estimated by a simple equation: the frequency of the Is gene is equal to the prevalence of the disease in the population divided by the penetrance of the illness in first-degree relatives with the HLA marker antigen, with the quotient further divided by two. This equation is valid if the gene is dominant and its gene frequency is low. An analysis of HLA data of 61 patients with dermatitis herpetiformis (DH) showed that the HLA-associated DH susceptibility gene is obviously dominant. The frequency of this DH susceptibility gene was estimated to be at least 0.0005 and the frequency of psoriasis susceptibility gene about 0.02 in Finland.