Down syndrome associated with a familial 14/21 translocation.

Down syndrome (mongolism) is one of the commonest chromosomal abnormalities in man with an incidence of between 1 and 2 per 1000 live births. In Northern Ireland the incidence is 1 in 6301. About 95 per cent have 47 instead of 46 chromosomes with an extra chromosome 21 (Trisomy 21). A small proportion , between 2 and 5 per cent are due to an unbalanced chromosomal translocation, usually involving the D(13-15) group chromosomes, particularly chr(nosomes 14 and 21. About half of the cases of translocation Down syndrome are inherited and thus, other family relatives may carry the chromosomal translocation and have a high risk of having affected children. The purpose of this paper is to describe our recent experience with a family which illustrates the importance of cytogenetic examination in all Down syndrome infants, of family follow-up when an unbalanced translocation is discovered, and of providing genetic counselling for those relatives who are carriers.