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Down Syndrome was first described in 1866, when John Langdon Down noted and described certain similar recurrent features in some of his patients with retarded development. We know that people with Down Syndrome have been born to parents of all nations and racial groups throughout history, because genetic, including chromosomal changes have been occurring continuously.

The basis of Down Syndrome was first explained in1959 by the French geneticist Jerôme Lejeune, whose team of researchers discovered that people with Down Syndrome have an extra 21st chromosome. No one has yet explained how Down Syndrome originates, but is certain is that no “blame” attaches to either of the parents. Of course high-risk activity by the mother during pregnancy (associated with alcohol, drugs, nicotine and poor nutrition) as well as certain diseases, can damage the child, but they do not result in Down Syndrome, which is caused by a faulty division of genetic material that occurs by chance. The extra chromosome can come from either the father or the mother, but most importantly it is a genetic accident for which neither of the parents is responsible.

In about 88% of the cases it consists of so-called free trisomy or non-disjunction, when the 21st chromosome migrates during cell-division just prior to fertilisation. A couple of 21st chromosomes to not divide during the first division. When the egg fuses with the sperm, i.e. after fertilisation, a new germ cell is created that now has 47 chromosomes. This fault then recurs during subsequent cell division, so that every cell of a person with Down Syndrome has 47 chromosomes instead of 46.

About 4.3% of cases are a translocational form of Down Syndrome, in which part of the genetic information of the 21st chromosome attaches itself to another chromosome, usually the 14th.

In about 7.7% of cases Down Syndrome are of the so-called “mosaic” variety, where the altered division occurs after fertilisation. The first division has the usual number of chromosomes (46) but faulty division occurs subsequently. In this type of DS, the individuals have some cells with the normal number of chromosomes and some with the additional 21st chromosome.
Down Syndrom
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