The separation of susceptibility to psoriasis from age at onset.

The prevalence of psoriasis among first-degree relatives of 1209 patients with severe psoriasis was studied by means of a questionnaire survey. Siblings of patients with an affected parent were more than 4 times as likely to have psoriasis as siblings of patients without an affected parent. Siblings of patients with onset before age 15 were more than 3 times as likely to have psoriasis as siblings of patients with onset after age 30. The increased prevalence associated with each factor was independent of the other factor. This relation suggests that determinants of susceptibility to psoriasis are separate from factors that influence age at onset in those who are susceptible. This separation is important because the genetics and biochemistry of susceptibility may be less complex than for age at onset. Prevalence among offspring was less than age-adjusted prevalence among siblings with an affected parent which suggests the major determinant of susceptibility is not a single dominant allele.