Humans have 23 pairs of chromosomes for a total of 46 chromosomes. Not just any combination of 46 will do. In order to develop and grow correctly, a person needs 46 of the right chromosomes. If a cell is missing just one chromosome, that cell is likely to die. If a cell has one too many, it too will probably die. An exact balance of chromosomes is important for growth and survival.
TWO KINDS OF CELL DIVISION
If sperm and eggs formed from the same kind of cell division as do all other body cells (all the cells in your body except for the sperm and egg), they would all contain 46 chromosomes. Think about what would happen then-46 chromosomes in an egg, 46 in a sperm. After fertilization (the sperm and the egg getting together), the new cell would contain 92 chromosomes. Even one extra chromosome makes cells die, and double the number kills it also. (Except in some plants.)
A second kind of cell division, sex cell division , is needed. It occurs only in specialized organs. In females sex cell division occurs in the ovaries. In males, it occurs in the testes. Sex cell division results in the formation of eggs and sperm.
HOW SEX CELL DIVISION TAKES PLACE
The important result of sex cell division is that the new cells have half as many chromosomes as did the original cell. If the original testis cell had 46 chromosomes, the new sperm will have 23 chromosomes. When the original cell divides, the chromosome pairs separate. This way, each new cell contains one chromosome from each pair. Sex cell division must be very exact. Only one of each of the 23 pairs of chromosomes must end up in an egg or sperm.
AN OCCASIONAL MISTAKE
Sometimes, there is a mistake in sex cell division. As a result, an egg or sperm may get too many or too few chromosomes. Usually this egg or sperm is not very healthy and soon dies. If it does survive and ends up in a fertilized egg, that egg may die or it may divide a few times and then die. When this happens, the woman probably will never know that she was pregnant. It is estimated that 70% of fertilized eggs are never carried to full term to be born alive.
Sometimes the fertilized egg not only divides but may survive for several weeks before it dies. In this case, the woman experiences a miscarriage.
An abnormal chromosome number is a common cause of miscarriages. Occasionally, however, a baby manages to survive all of the problems that go along with an abnormal number of chromosomes. For example, children who have an extra chromosome 21 have a total of
47 chromosomes. They have a condition known as Down Syndrome . (See the karyotype on the back of this page.) A syndrome means that there are many problems with the body. These children usually have some degree of cognitive disabilities and may have heart problems, ear problems, intestinal problems-problems with just about every system in their bodies. All because of one extra chromosome in every one of the trillions of cells of their body.

The reason why some cells with an extra chromosome 21 survive is probably because that chromosome is so small. You would never find a cell, let alone a person, with an extra chromosome 1. That chromosome is just too big. If a cell had an extra number 1, too much information would be off balance and the cell could not survive.
A Karyotype of a person with Down Syndrome. Notice that there are three chromosome
21s.
There are a few other syndromes that are caused by an abnormal number of chromosomes. One is called Turner Syndrome . These females have 45 chromosomes because they only have one X chromosome and no other sex chromosomes. These females are sterile, they cannot have children. They are usually short and have other problems with their bodies.
In
Klinefelter’s Syndrome, two X chromosomes and a Y chromosome are present for a total of 47 chromosomes. This person is a male and does not have any major problems with his body.