NORMAL CELL DIVISION

Human cells contain 46 chromosomes which carry the estimated 70,000 genes needed for normal growth and development. There are 23 pairs: 22 pairs of autosomes, and two sex chromosomes, X and Y (XX in females, XY in males). Normal female and male karyotypes are abbreviated to 46XX and 46XY respectively.

Cells divide as the body grows. In this process (mitosis) the chromosomes replicate into two identical copies and produce identical daughter cells. The other method of cell division is called meiosis and occurs in the formation of gametes in the ovaries and testes. Meiosis occurs in two stages during which there is an exchange of genetic material between pairs of chromosomes, and a reduction of the number of chromosomes in each cell to 23 (one of each pair). The diploid number is regained when the egg and sperm fuse at fertilisation.

TRISOMY

In cases of trisomy an additional chromosome is present over and above the usual pair. The condition is almost always caused by either the sperm or the egg containing a pair of chromosomes (rather than a single chromosome). This arises from a failure of the normal separation of a pair of chromosomes during cell division, if it occurs during meiosis the resulting gamete will have 24 chromosomes and the complementary gamete with 22 chromosomes will be non-viable.
Autosomal trisomies are more common with increasing maternal age. This probably reflects the differences between male and female gametogenesis. In particular the primary oocyte ages while in "suspended animation" for up to 50 years within the ovary. Trisomies of any chromosome can occur during gametogenesis, but the majority of resulting pregnancies are not viable and result in early miscarriage.