Psoriasis Genetics — SCE Dermatology MCQ

Correct answer: E — Approximately 10-15% (rising to approximately 40-50% if both parents are affected)

The correct answer is E, approximately 10-15% (rising to approximately 40-50% if both parents are affected). Psoriasis is a polygenic, multifactorial disease rather than a single-gene Mendelian trait, so risk to offspring is expressed as an empirical probability rather than a fixed Mendelian ratio. Family and twin studies show that risk scales with the number of affected first-degree relatives: a child with one affected parent carries roughly 10 to 15% lifetime risk, increasing to around 40 to 50% when both parents have psoriasis, reflecting cumulative loading of susceptibility loci (notably HLA-Cw6) combined with environmental triggers needed to unmask disease. This gene-environment interaction explains why monozygotic twin concordance is high but well below 100%, confirming incomplete penetrance rather than simple dominant or recessive inheritance. Why the other options are wrong: D. 50% (autosomal dominant): Psoriasis does not follow single-gene autosomal dominant inheritance; no individual gene confers a fixed 50% transmission risk, and penetrance is markedly incomplete. B. 25% (autosomal recessive): There is no recessive inheritance pattern in psoriasis; risk does not require two copies of a single mutant allele, and the actual one-parent risk is lower than this figure implies. A. 0%: Family history is one of the strongest recognised risk factors for psoriasis, so risk to offspring of an affected parent is clearly above baseline population prevalence. C. 100%: Even with two affected parents or a monozygotic twin with psoriasis, disease expression is not guaranteed, confirming incomplete penetrance and a requirement for environmental triggers. Key point: Psoriasis shows polygenic, multifactorial inheritance with incomplete penetrance, giving an empirical offspring risk of about 10-15% with one affected parent and 40-50% with two, not a Mendelian dominant or recessive pattern.

Reference: British Association of Dermatologists, Patient Information Leaflet: Psoriasis (causes and genetics section), www.bad.org.uk/pils/psoriasis