Translocation Down syndrome

About 5% of cases of Down syndrome are due to translocation,
in which chromosome 21 is translocated onto chromosome 14
or, occasionally, chromosome 22. In less than half of these cases
one of the parents has a balanced version of the same
translocation. A healthy adult with a balanced translocation
has 45 chromosomes, and the affected child has 46
chromosomes, the extra chromosome 21 being present
in the translocation form. The risk of Down syndrome in
offspring is about 10% when the balanced translocation is
carried by the mother and 2.5% when carried by the father. If
neither parent has a balanced translocation, the chromosomal
abnormality in an affected child represents a spontaneous,
newly arising event, and the risk of recurrence is low (1%).
Recurrence due to parental gonadal mosaicism cannot be
completely excluded.