A 38-year-old woman delivers a baby at term with hypotonia, flat facial profile, single palmar crease, and upslanting palpebral fissures. Karyotype shows free trisomy 21. The nondisjunction event leading to this karyotype occurs most commonly during:

• A Maternal meiosis II
• B Maternal meiosis I ✓
• C Paternal meiosis I
• D Early embryonic mitosis

Explanation

Approximately 95% of Down syndrome cases result from meiotic nondisjunction, and about 95% of these occur in the mother, predominantly in meiosis I. Advanced maternal age is the strongest risk factor because the long prophase of oocyte meiosis I, arrested from fetal life until ovulation, predisposes to nondisjunction. Paternal nondisjunction accounts for under 5% of cases, and mitotic nondisjunction after fertilization produces mosaic trisomy 21, which is a different cytogenetic category.

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.