The Genetics Behind Down Syndrome

Down syndrome is chromosomal abnormality and genetic disease that is caused by three major factors. These include genetics, chromosome formation, and cell division. The most common cause of the formation of Down syndrome is genetics. Normally the nucleus of each cell contains 23 pairs of chromosomes. Half of these 23 chromosomes are inherited from each parent. Genetics are described as being grouped along the chromosomes. The chromosomes are the units of genetic information that exists within every cell in the body. In Down syndrome, the cells contain 47 chromosomes rather than the normal 46 chromosomes. The extra chromosome is the number 21. This genetic form of Down syndrome is referred to as Trisomy 21 because rather than two copies of the 21st chromosome, there are three copies of it. Having the extra 21st chromosome in actuality adds genetic material to the cells and thus, alters the pattern of development in the fetus. As the embryo continues to develop, the extra chromosome 21 is replicated in every cell in the body.

Chromosomal abnormalities can occur in one of two ways. A mutation may be inherited from one of the individual's parents. A mutation may also occur for the first time in a single gamete or during the process of fertilization between an egg cell and a sperm cell. The involved person will have this mutation in all of his or her cells in the body and in most cases will be able to pass the mutation on to any offspring. In this case, the parents are not involved and are not considered "carriers" of a chromosomal abnormality.

Down syndrome is the most frequent trisomy found in humans. It is also the most common condition encountered in genetic counseling. The incidence of Down syndrome is commonly associated to extended maternal age. The genetic principles behind this theory are thought to be found in the formation of oocytes, which are the cells from which the female eggs develop. In females, the oocytes are formed before birth and lay dormant in the ovary until ovulation. In the case of older mothers, it is feasible that an oocyte can be in this condition within the ovary for forty years or more. During this time, the oocyte can also be subjected to environmental factors that have the potential for causing additional chromosomal mutations. When the Down syndrome develops as a result of the male sperm, the event is called nondisjunction. This occurs when an accident happens in the development of a sperm cell, which causes it to have 24 chromosomes rather than the normal 23 chromosomes.

Another rare genetic accident that occurs in the formation of Down syndrome is translocation. During cell division, the chromosome 21 in all actuality somehow breaks. A piece of the now broken chromosome 21 becomes attached to another chromosome. Each cell still has its 46 chromosomes – 23 from each parent – but the extra piece of the 21st chromosome results in the signs and symptoms of Down syndrome. Translocations are so rare that they occur in only 3% to 4% of all cases of Down syndrome.

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