A 38-year-old woman delivers a term infant with flat facies, upslanting palpebral fissures, single palmar crease, and a systolic murmur. Karyotype shows 47,XX,+21. Which mechanism accounts for the vast majority of such conceptions?

• A Paternal uniparental disomy of chromosome 21
• B Robertsonian translocation inherited from a carrier parent
• C Postzygotic mitotic nondisjunction producing mosaicism
• D Meiotic nondisjunction in maternal gametogenesis ✓

Explanation

About 95 percent of full trisomy 21 results from nondisjunction during maternal meiosis, usually meiosis I, and its frequency rises sharply with maternal age due to prolonged arrest of oocytes in prophase I. Translocations account for roughly 4 percent and mosaicism about 1 percent. The presence of complete trisomy in every cell plus advanced maternal age makes meiotic nondisjunction the answer; a Robertsonian translocation would show 46 chromosomes with a der(14;21) on karyotype.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.