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What is Down Syndrome?

Chromosomes of a boy with Down Syndrome
The chromosomes of a boy with Down Syndrome. The arrow points to the extra chromosome 21.

What are chromosomes?

Our chromosomes contain the genetic code which controls and instructs cell division, growth, and function. They are the structures inside the nucleus of living cells that contain hereditary information. A person normally has 46 chromosomes, 23 inherited from each parent. Every person has a unique genetic code (with the exception of identical twins). It is this uniqueness which makes the physical appearance of each person different.

How do we get our chromosomes?

Our chromosomes are given to us by our parents. Each parent gives us 23 chromosomes which come from the egg and sperm cells. When they combine, they produce a cell with 46 chromosomes. This cell then divides, the result being that every cell contains identical genetic material.

What is Down syndrome?

Down syndrome is caused by a person having three copies of chromosome 21 instead of two copies. This is why Down syndrome is also referred to by the name Trisomy 21. The picture above shows a genetic analysis of a person with Down syndrome. You will notice that the arrow is pointing to the set of number 21 chromosomes. Instead of a pair of chromosomes, as is found in all the other chromosomes, number 21 includes three chromosomes. It is important to understand that all of the chromosomes of this person are normal. It is the fact that there is an extra chromosome that causes Down syndrome. Every cell in a person with Down syndrome will contain 47 chromosomes (with the exception of Mosaic Down syndrome which is discussed below) instead of 46 chromosomes. There are many theories about how the extra chromosome causes the effects of Down syndrome but little is currently known. Research, however, is continuing and a breakthrough may provide possible treatments to lessen the effects.

How does this happen?

During cell division to create a germ cell (either sperm or egg), a cell containing 46 chromosomes divides into two germ cells each containing 23 chromosomes. Sometimes this division does not happen properly and one cell may contain 22 chromosomes and the other may contain 24 chromosomes. This can happen if the chromosomes do not properly separate and instead "stick together." This is called nondisjunction because the chromosomes have failed to disjoin or split-up. 75% of the time it is the egg cell which carries the additional chromosomal material, 25% of the time it is the sperm cell. If the cell containing 24 chromosomes combines with a cell containing 23 chromosomes, the new cell will contain 47 chromosomes instead of 46. If the trisomy is chromosome 21, the person will have Down syndrome. Other conditions arise if the duplicated chromosome is a different chromosome. If the trisomy is chromosome 13, the person will have Patau's syndrome. If the trisomy is chromosome 18, the person will have Edward's syndrome. These conditions are rarer than Down syndrome and have their own characteristics which are different than Down syndrome.

Are there different types of Down syndrome?

There are three different types of Down syndrome: Standard Trisomy 21, Translocation, and Mosaicism.

Standard Trisomy 21 is when the extra chromosome 21 comes from either the egg or sperm cell. Between 90% and 95% of all Down syndrome is Standard Trisomy 21.

Translocation is caused when a piece of chromosome 21 is located on another chromosome such as chromosome 14. The person with Translocation Trisomy 21 will have 46 chromosomes but will have the genetic material of 47 chromosomes. The person with Translocation Trisomy 21 will exhibit all the same characteristics of a person with Standard Trisomy 21 since they have three copies of chromosome 21. Translocation occurs between 3% and 5% of cases of Down syndrome.

Mosaicism is when a person has a mix of cells, some containing 46 chromosomes and some containing 47 chromosomes. This occurs either because: a) The person received 46 chromosomes at fertilization but somewhere during early cell division the chromosome 21 cell pairs failed to split creating a cell with 47 chromosomes and a cell with 45 chromosomes. The cell with 45 chromosomes can not survive but the cell with 47 chromosomes will continue to divide. All cells that come from this cell will contain 47 chromosomes. b) The person received 47 chromosomes at fertilization but later during cell division the extra chromosome is lost. Mosaicism occurs in 2% to 5% of cases of Down syndrome. A person with Mosaic Down syndrome may exhibit all, some, or none of the characteristics of Down syndrome depending on the percent of cells carrying the extra chromosome and where these cells are located.

Is Down syndrome inherited?

The vast majority of cases of Down syndrome are not inherited. Only in cases of Translocation Down syndrome and then in only 1 of 3 cases of this type of Down syndrome is the condition inherited. These inherited cases occur because one of the parents is a carrier. A carrier will have 45 chromosomes instead of 46 but they will have all the genetic material of a person with 46 chromosomes. Remember that in Translocation Down syndrome the extra chromosome 21 material is located on a different chromosome. A carrier will have the extra material but will have only one chromosome 21. The carrier will not exhibit any of the symptoms of Down syndrome because they have the correct amount of genetic material.

A carrier will have an increased chance of having a child with Down syndrome. If the carrier is the mother, the chances are approximately one in five of having a child with Translocation Down syndrome. If the carrier is the father the odds are reduced to between one in twenty to fifty. In cases where the carrier has no unattached chromosome 21, all the carriers children will have Down syndrome. In all cases of Down syndrome but especially in cases of Translocation Down syndrome, it is important that the parents have genetic counseling to determine their risk.

Did I cause my child to have Down syndrome?

Simple answer: NO! There is nothing you can do to prevent Down syndrome and nothing you can do to change your odds of having a child with Down syndrome. Down syndrome occurs in all races, in all socio-economic conditions, and in all countries. No relationship between diet or illness and Down syndrome has ever been found. The only established relationship is with maternal age. A woman is more likely to have a child with Down syndrome as she grows older with a sharp rise in risk at 35-40 years of age. However, 80% of all children with Down syndrome are born to mothers under 35.

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This page last updated on 15 April 2008.

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