The Genetics behind Down Syndrome

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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