Down syndrome is caused by a genetic anomaly involving an extra copy or part of chromosome 21. Normally, humans have 23 pairs of chromosomes, but in Down syndrome, there is an additional full or partial chromosome 21, which leads to developmental and physical changes. There are three main types of genetic causes for Down syndrome:
- Trisomy 21 (about 94-95% of cases): This is the most common form, caused by an extra full copy of chromosome 21 in all cells. It usually results from a failure of chromosome 21 to separate properly during the formation of egg or sperm cells, a process called nondisjunction. This error is mostly maternal in origin and occurs randomly with no environmental or behavioral causes known. The chance of this happening increases with maternal age.
- Translocation Down syndrome (about 4% of cases): Part of chromosome 21 breaks off and attaches to another chromosome. This extra piece causes the features of Down syndrome. Translocation can sometimes be inherited from a parent who carries a balanced translocation but does not have Down syndrome themselves.
- Mosaic Down syndrome (about 2% of cases): Only some cells have the extra chromosome 21, due to an error in cell division that occurs after fertilization during early embryo development. Individuals with mosaic Down syndrome have a mixture of normal cells and cells with trisomy 21.
The extra genetic material from chromosome 21 causes overexpression of genes in that chromosome, which leads to the characteristics associated with Down syndrome. Most cases are not inherited but occur as a random event during reproductive cell formation or early fetal development.
