Table 1.
Different origins of Down syndrome (trisomy 21).
| Form of DS | Standard trisomy 21 (nondisjunction) | Robertsonian trisomy 21 | Mosaic trisomy 21 | Partial trisomy (21q22.3) |
|---|---|---|---|---|
| Frequency | 95% | 4% | 3-5% | 1% |
| Mechanism | Maternal meiosis 85-90% (i) Meiosis I (75%) (ii) Meiosis II (25%) |
Familial 25% | Mitotic | — |
| Mechanism | Parental meiosis 3-5% (i) Meiosis I (25%) (ii) Meiosis II (75%) |
De novo 75% | — | — |
| Chromosomal analysis | 47 XX+21 47 XY+21 |
46 XX/XY Robertsonian translocation between D and G chromosomes (q10; q10)+21 46XX/XY+21, i (21) (q10) |
47 XX+21 47XY+21 46 XX 46 XY |
46 XX/XY, duplication (21) (q22.3) |