Recurrence risks for Down syndrome.

The recurrence risks for Down syndrome due to an inherited translocation are estimated from empirical data in the literature for two maternal age groups: mothers under 30 and mothers 30 and over. These risks were found to be approximately 0.3% and 0.05%, respectively. The probability for two Down syndrome sibs both having an inherited translocation was estimated as about 18.2% for the former age group and 2.7% for the latter. The relative effectiveness in preventing. Down syndrome births by karyotyping affected children is discussed.