In Down syndrome, what is the most common mechanism of chromosomal abnormality accounting for approximately 95% of cases?

Explanation

## Chromosomal Mechanisms in Down Syndrome **Key Point:** Regular trisomy 21 resulting from non-disjunction during meiosis accounts for approximately 95% of Down syndrome cases and is the most common mechanism. ### Types of Down Syndrome and Their Frequencies | Type | Mechanism | Frequency | Karyotype | Inheritance Pattern | |------|-----------|-----------|-----------|---------------------| | **Regular Trisomy 21** | Non-disjunction during meiosis I or II | ~95% | 47,XX,+21 or 47,XY,+21 | Sporadic; risk increases with maternal age | | **Robertsonian Translocation** | Fusion of chromosome 21 with another acrocentric chromosome (usually 14) | ~3–4% | 46,XX,der(14;21) or 46,XY,der(14;21) | Can be inherited from carrier parent; recurrence risk 10–15% if mother is carrier, 3% if father is carrier | | **Mosaic Down Syndrome** | Non-disjunction during early mitosis post-fertilization | ~1–2% | Mix of 47,+21 and 46,21 cell lines | Sporadic | | **Isochromosome 21q** | Rare structural variant | <1% | 47,+i(21q) | Sporadic | ### Non-Disjunction: The Primary Mechanism **Mnemonic:** **MII-ND** = Meiosis II Non-Disjunction is the most common specific event - Failure of sister chromatids to separate during meiosis II accounts for ~75% of regular trisomy 21 cases - Meiosis I non-disjunction accounts for ~25% of cases - The error occurs almost exclusively during oogenesis (maternal origin in >90% of cases) ### Maternal Age Effect - Risk at maternal age 20: approximately 1 in 1,500 - Risk at maternal age 35: approximately 1 in 350 - Risk at maternal age 45: approximately 1 in 30 - This reflects the prolonged arrest of oocytes in prophase I, increasing the risk of non-disjunction with advancing maternal age **High-Yield:** Regular trisomy 21 from non-disjunction is sporadic and not inherited; however, Robertsonian translocation Down syndrome can be inherited from a carrier parent, making genetic counseling crucial for family planning. **Clinical Pearl:** Mosaic Down syndrome may present with milder phenotypic features and less severe intellectual disability compared to regular trisomy 21, as only a proportion of cells carry the extra chromosome 21.