Which type of mutation causes Down syndrome?
Answers
The most common form of Down syndrome is called trisomy 21. This is a condition wherein people have 47 chromosomes in each cell instead of 46. An error in cell division called nondisjunction causes trisomy 21. This error leaves a sperm or egg cell with an extra copy of chromosome 21 before or at conception.
Down S. is caused by the presence of all or part of an extra (3rd) copy of chromosome # 21.
If this happened before conception (i.e.:the egg or the sperm has the extra chromosome), all the cells in the body have this extra genetic information.
Sometimes, this can happen very early in the development of a zygote during cell duplication (mitosis). In this case, the cells not affected continue developmental normally.
Of the 2 cells affected, the one with the extra 21 st chromosome continues to give rise to Down syndrome cells, while the one missing its 2nd copy dies off.
This results in a percent of cells coding for Down syndrome, & is known as mosaicism.
Backstory: genetic material on the chromosomes codes for production of proteins, hormones, etc and controls when and how a cell does its job. Excess production or inhibition of some of these will upset balance and, in this case, result in the syndrome known as Down.
More info can be found at NDSS website - National Down Syndrome Society.