Chromosome abnormalities usually occur when there is an error in cell division. There are two kinds of cell division.
- Mitosis results in two cells that are duplicates of the original cell. In other words, one cell with 46 chromosomes becomes two cells with 46 chromosomes each. This kind of cell division occurs throughout the body, except in the reproductive organs. This is how most of the cells that make up our body are made and replaced.
- Meiosis results in cells with half the number of chromosomes, 23 instead of the normal 46. These are the eggs and sperm.
In both processes, the correct number of chromosomes is supposed to end up in the resulting cells. However, errors in cell division can result in cells with too few or too may copies of a chromosome. Errors can also occur when the chromosomes are being duplicated.
Other factors that can increase the risk of chromosome abnormalities are:
- Maternal Age: Women are born with all the eggs they will ever have. Therefore, when a woman is 30 years old, so are her eggs. Some researchers believe that errors can crop up in the eggs' genetic material as they age over time. Therefore, older women are more at risk of giving birth to babies with chromosome abnormalities than younger women. Since men produce new sperm throughout their life, paternal age does not increase risk of chromosome abnormalities.
- Environment: Although there is no conclusive evidence that specific environmental factors cause chromosome abnormalities, it is still a possibility that the environment may play a role in the occurance of genetic errors.
Source: http://www.genome.gov/11508982
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