ABSTRACT
It has been known for many years that psoriasis has a hereditary basis, as it was noted that the disease tended to run in families. The early observations supporting an inherited factor were derived from family studies. One of the earliest was in a closed community in a small town in Germany, conducted over a 30-year period. It was found at the beginning of the study that 34% of patients with psoriasis had a positive family history; 5 years later this number had increased to 39%, and at the end of the 30 years it was 56%8. Some studies of families with psoriasis through three and four generations strongly support the concept of Mendelian dominance with incomplete penetration, as every affected child had an affected parent. However, other family studies through a similar number of generations did not find that all affected individuals with psoriasis had an affected parent, and asymptomatic carriers were assumed. These latter studies considered that the inheritance was autosomal recessive with 90% penetration. The more recent studies have suggested that psoriasis is a polygenic disorder, i.e. several genes are involved in the pathogenesis. In addition, the etiology of psoriasis is multifactorial, i.e. with environmental as well as genetic factors interacting to produce the manifestations of the disease. This interaction and the involvement of several genes is now referred to as a complex multifactorial disease.
