Which condition is a GENETIC (chromosomal) abnormality that is NOT inherited from the parents?

Correct answer: C. Turner syndrome

• A. DENTINOGENESIS IMPERFECTA is autosomal dominant (DSPP gene) with high penetrance and a clear family history — inherited.
• B. HAEMOPHILIA A is X-linked recessive, classically passed from carrier mother to affected son — inherited.
• C. CORRECT. TURNER SYNDROME (45,X) arises from a sporadic NONDISJUNCTION event during gametogenesis or an early post-zygotic error. It is unquestionably genetic — an entire sex chromosome is missing — yet it is NOT inherited, and recurrence risk for the parents is not increased. Features include short stature, webbed neck, primary amenorrhoea and ovarian dysgenesis, coarctation of the aorta, and orally a high-arched narrow palate, micrognathia and increased root resorption.
• D. CYSTIC FIBROSIS is autosomal recessive, transmitted through the parents' CFTR mutations — inherited by definition.

Note: The same distinction applies to DOWN SYNDROME (trisomy 21) and KLINEFELTER SYNDROME (47,XXY): all three are chromosomal, sporadic and non-inherited, with maternal age the main risk factor for the trisomies. Contrast 'genetic' (involving DNA/chromosomes) with 'hereditary' (passed down a family line) — every hereditary condition is genetic, but not every genetic condition is hereditary.