E. Non-dysjunction – Explanation
Down’s syndrome: epidemiology and genetics
Risk of Down’s syndrome with increasing maternal age
- risk at 30 years = 1/1000
- 35 years = 1/350
- 40 years = 1/100
- 45 years = 1/30
One way of remembering this is by starting at 1/1,000 at 30 years and then dividing the denominator
by 3 (i.e. 3 times more common) for every extra 5 years of age
Cytogenetics
| Mode | % of cases | Risk of recurrence |
|---|---|---|
| Non-disjunction | 94% | 1 in 100 if under mother < 35 years |
| Robertsonian translocation (usually onto 14) |
5% | 10-15% if mother is translocation carrier 2.5% if father is translocation carrier |
| Mosaicism | 1% |
The chance of a further child with Down’s syndrome is approximately 1 in 100 if the mother is less
than 35 years ld. If the trisomy 21 is a result of a translocation the risk is much higher
