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Each of the 100 trillion cells in the human body (except for red blood cells) contains the entire human genome. Chromosomes are the string-like elements within the nucleus (center) of every cell of your body. They contain genetic information made of DNA. A gene occupies a specific location on a chromosome. Normally, there are 23 identical pairs of chromosomes (6 feet of DNA) in each cell, with a total of 46 chromosomes. Each partner normally provides 23 chromosomes during fertilization. If an egg or sperm have an abnormal chromosome package, the embryo that they create will have a chromosomal abnormality as well. This is sometimes due to a rearrangement of chromosomes, or a missing piece of a chromosome. In some cases there is a missing chromosome, or an extra chromosome (aneuploidy) leading to an inheritable disorder. Any embryo with a missing chromosome (monosomy) will stop growing before implantation (fatal anomaly). If the aneuploidy involves chromosomes including 13, 18, 21, X or Y, the pregnancy may go to term. The most common of these non-fatal anomalies is trisomy 21, or Downs syndrome, in which an extra chromosome 21 is present. Others include Turner’s syndrome in females and Klinefelter’s Syndrome in males.
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