A 28-year-old Indian man presents with infertility and is found to have tall stature with disproportionately long legs, gynecomastia, and small firm testes (volume <6 mL). Hormonal evaluation shows low testosterone, elevated FSH and LH, and elevated estradiol. Karyotype analysis is performed. The structure marked **A** in the diagram represents the extra X chromosome in the 47,XXY pattern. Which of the following best explains the pathogenesis of this chromosomal abnormality?

Explanation

## Why Meiotic nondisjunction during gametogenesis is right The extra X chromosome marked **A** in Klinefelter syndrome (47,XXY) arises from meiotic nondisjunction occurring in approximately 50% of cases during paternal meiosis I and 50% during maternal meiosis I or II, with maternal cases showing correlation with advanced maternal age. This failure of chromosome segregation during meiosis results in gametes with either two X chromosomes (in maternal nondisjunction) or one X and one Y chromosome (in paternal nondisjunction), which upon fertilization produce the 47,XXY karyotype. This is the established pathogenic mechanism described in Harrison Principles of Internal Medicine and Williams Endocrinology. ## Why each distractor is wrong - **Mitotic nondisjunction during early embryonic development**: While mitotic nondisjunction can produce mosaicism (46,XY/47,XXY), the classic Klinefelter syndrome with uniform 47,XXY karyotype in all cells results from meiotic, not mitotic, nondisjunction. Mitotic errors occur post-fertilization and would create mosaic patterns. - **Unequal crossing over between homologous chromosomes**: Unequal crossing over produces duplications and deletions of chromosome segments, not the addition of an entire extra chromosome. This mechanism is responsible for structural rearrangements, not numerical aneuploidy like XXY. - **Robertsonian translocation involving the X chromosome**: Robertsonian translocations involve acrocentric chromosomes (13, 14, 15, 21, 22) and result in structural rearrangement with a reduced total chromosome count, not the numerical increase seen in 47,XXY. **High-Yield:** Klinefelter syndrome (47,XXY) — the most common sex chromosome aneuploidy and most common genetic cause of male infertility — results from meiotic nondisjunction, with the extra X chromosome marked **A** being the pathognomonic finding. [cite: Harrison Principles of Internal Medicine 21e Ch 401; Williams Endocrinology 14e]