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Genetic Forms of DS
All individuals with DS have extra chromosome 21 material. There are 3 genetic mechanisms for trisomy 21. The first and most common, is called non-disjunction, where there is an entire extra chromosome 21 in all cells. A chromosome study (karyotype) of trisomy 21 is shown in Fig. 1.
The second is mosaic DS, where trisomy 21 cells are mixed with a second cell line, usually "normal" (46,XX or 46,XY). Individuals with this form of DS are frequently a bit milder in their presentation, depending on the proportion of normal cells.
The third is a translocation DS, about 3-5% of the total, where part or all of chromosome 21 is translocated to another chromosome, usually 14. Translocation DS does not vary with age. Children with translocation DS are indistinguishable from individuals with the usual form of DS.
Trisomy 21
An extra whole chromosome 21 in all cells examined is found in about 92 per cent of all DS individuals. This is shown in Figure 1. DS is common......

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