Chromosomal abnormalities occur in around 1 in 200 births and account for up to half of all miscarriages.
As a cell gets ready to divide, chromosomes are separated into each of the dividing cells (see what is cell division?) through proteins that bind and physically separate and move.
A failure in this chromosome distribution during the process of cell division, in the making of gametes, results in sperm or oocytes lacking, or gaining, a chromosome. At fertilization these will produce cells, and subsequent embryos, containing only a single chromosome or three chromosomes (trisomy) instead of the usual pair.
These numerical chromosomal abnormalities tend to occur more in females during oogenesis (the production of oocytes), and the chance of this occurrence increases with maternal age; it is suspected that the aged molecular apparatus in these cells, involved in the cell division, leads to mistakes in chromosome separation as a cell divides.
Although extra numbers of any chromosome can occur, only extra copies of the X and Y chromosomes (Klinefelter’s, Triple X and XYY syndromes), the 21st (Down’s syndrome) and, to a smaller extent, the 18th and 13th chromosomes, have any compatibility with life. A foetus lacking any chromosome of a pair, apart from the X chromosome (Turner’s syndrome), is unable to survive.