A population genetic study of psoriasis.

We present epidemiological data for 5197 families with psoriasis. Errors in reporting have been analysed. Analysis of the data provides indications of random mating with respect to whether the partner has the skin disease or not. Data on psoriasis among parents, siblings and children are provided, and particular attention has been paid to the age at onset of psoriasis. Psoriasis was present in the parents of about 36% of the probands. In families in which one or both parents have psoriasis, the occurrence of the disease among the siblings does not provide any information which differentiates between a dominant and recessive mode of inheritance, but is compatible with both. In families in which neither parent had psoriasis, provided there was a proband with psoriasis, the probability of the siblings suffering from psoriasis was close to 0.25, indicating a recessive mode of inheritance. The distribution of psoriasis among the parents of all probands, and among the children of probands, was also compatible with this mode of inheritance. The prevalence of psoriasis in the elderly was estimated to be about 5%, and the gene frequency in the whole population 25%. These findings show that, with regard to first-degree relatives, the inheritance of psoriasis can fit an autosomal recessive model. The concept of a recessive mode of inheritance may be used as a basis for genetic counselling.