Normally, each cell of a person contains 46 chromosomes; each chromosome contains a variety of genes which dictate our appearance, intelligence, and skin colour. Abnormal chromosomes can lead to the possibility of Down Syndrome. Changes in chromosomes are mainly from three categories:
– Trisomy 21
Trisomy 21 is caused by the presence of an extra copy of chromosome 21; the gamete now has 24 chromosomes. When combined with a normal gamete from the other parent, the embryo then has 47 chromosomes. Trisomy 21 is the cause of approximately 95% of observed cases of Down Syndrome.
– Robertsonian translocation
The long arm of chromosome 21 is attached to another chromosome, often chromosome 14 or chromosome 21. This is the cause of 5–6% of observed cases of Down Syndrome.
When some of the cells in the body are normal and other cells have trisomy 21, we call it mosaic Down Syndrome (46 or 47 chromosomes). This case is rare and is the cause of only 1–2% of observed cases of Down Syndrome.