My sister has a two yrs old down syndrome boy.what is the chance that a second baby is also affected?is there chance to reduce the risk?

>90% of DS is caused: By an error in cell division in which an egg or sperm cell accidentally gains an extra copy of chromosome (chr.) 21 before joining the other to form a fertilized egg. Both parents can be tested for a balanced translocation, rearrangement of genetic material between a chr.21 & another chr. That has a 3% risk if from dad & a 12% risk if from mom of becoming imbalanced if passed on to their child.

Depends: Traditional ds (>85%) have an extra 21 chromosome & recurrence risk runs 1% for young mothers but 1% + the age factor as a women ages. There are uncommon forms of ds due to transfer of pieces between chromosome where the mom or dad could be an asymptomatic carrier. The % of those varies with the defect & it would be best to consult a geneticist. If regular ds, the numbers above apply.