Down syndrome (genetics)
Down syndrome is caused by the presence of extra genetic material from a chromosome 21. The following table illustrates the three different types:-
The risk of having a child with Down syndrome increases as a mother gets older.
| Form | Inheritance | Percentage of Down syndrome cases | Cause |
|---|---|---|---|
| Full trisomy 21 | Not inherited | 95% | Nondisjunction, extra chromosome comes from the mother 88% of the time, and from the father 12% of the time |
| Translocation Down syndrome | Inherited | 2-3% | Balanced translocation |
| Mosaic Down syndrome | Not inherited | 2-3% | Occurs as a random event during cell division early in fetal development |
The risk of having a child with Down syndrome increases as a mother gets older.
- Maternal age of 35 years, the risk is 1 in 385
- Maternal age of 40 years, the risk is 1 in 106
- Maternal age of 45 years, the risk is 1 in 30
