The mode of inheritance of the susceptibility to psoriasis vulgaris (PV) was investigated by segregation analysis. Because our main aim was to search a possibility of the double recessive hypothesis for the disease susceptibility, we examined 83 families of normal × normal mating by single selection out of 93 probands, a number sufficient to be analyzed genetically. Nine of the 83 couples had two or more affected children, including probands. As to the 83 families, the hypothesis of the single recessive model for PV was ruled out, but the double recessive model allowing for 62 to 80% of sporadic cases was fitted to the family data. Penetrance value of the disease trait ranged from 33 to 43%. It is thus likely that about 20 to 38% of patients with psoriasis are homozygous for two unlinked autosomal recessive genes.
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