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Chromosomes are parcels of DNA, large enough to be seen under a microscope. There are usually 46 (23 pairs) in each cell of our body. Out of these, 22 pairs are autosomes and 1 pair are sex chromosomes. We inherit 23 from our mother and 23 from our father.
A normal compliment of chromosomes is known as a karyotype and the most severe medical problems are caused by the either the loss or gain of whole chromosomes, as this affects hundreds of genes. Following are a few examples of some of the conditions caused by chromosomal abnormalities.
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This is caused by the addition of genetic material, and in Down's syndrome that's a whole extra chromosome. It happens in cell division (meiosis) during the production of the gametes. Meiosis takes place in the ovaries or testes and 1 cell with 46 chromosomes splits into 2 cells with 23 chromosomes each. This is how sperm and egg cells are produced.
During cell division each pair of chromosomes should separate and travel to specific parts of the dividing cell. This is known as 'disjunction.' However, sometimes the pair does not divide and instead sticks together and travels to its place in the dividing cell. This is known as 'non-disjunction' and the result is that one cell (gamete) will have 24 chromosomes and the other 22.
If such a gamete is used in fertilization, its 2 chromosomal copies are united with 1 chromosomal copy from the normal gamete, which results in an embryo with three copies of that particular chromosome. People with Down's syndrome have an additional copy of chromosome 21.
The effects are different from person to person but include mental retardation, heart defects, a short stature and decreased muscle tone.
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This genetic disorder is caused by an additional X chromosome, resulting in a karyotype of 47. Males with this condition will still have 1 Y chromosome, but in addition they'll possess 2 X chromosomes when they should have 1. The extra X can either come from the father's sperm or the mother's egg.
Boys and men with Klinefelter's Syndrome express a variety of symptoms including, breast enlargement, lack of facial and body hair, and they tend to be tall. Such individuals also tend to be infertile.
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This is due to the absence of one of the X chromosomes. The missing sex chromosome error can occur in either the mother's egg or the father's sperm; however, it usually occurs because the father's sperm lacks the X chromosome.
Women with this condition present a range of symptoms, and no two individuals are alike, but common characteristics of the condition include rudimentary ovaries, short stature and dry eyes.
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Currently there are no permanent cures for chromosomal abnormalities. However, treatments are available to mitigate the worst effects of some of the symptoms, allowing patients to live a much happier life. At the same time scientists are trying to learn about the genetic basis of these diseases to find out exactly what goes wrong during early cell division.