Twin studies in schizophrenia show a concordance rate of approximately 50 percent in monozygotic twins and about 10 to 15 percent in dizygotic twins. What does this pattern establish?
- A Genetic factors are important but incompletely penetrant, implicating additional nongenetic influences ✓
- B Shared family environment alone explains transmission
- C Schizophrenia is determined entirely by a single autosomal recessive gene
- D Concordance data prove schizophrenia is purely a neurodegenerative disease
Correct answer: A. Genetic factors are important but incompletely penetrant, implicating additional nongenetic influences
Explanation
If genetics were sufficient, monozygotic concordance would approach 100 percent; if environment alone mattered, monozygotic and dizygotic rates would be equal. C 50 percent monozygotic versus 10 to 15 percent dizygotic gap shows strong genetic loading with incomplete penetrance, leaving roughly half of monozygotic pairs discordant due to nonshared environmental factors such as obstetric complications, infection, and cannabis use. This supports a multifactorial polygenic model, not single-gene inheritance.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
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