Down syndrome is a genetic condition that occurs as a result of an extra chromosome (chromosome 21).
The human body is made up of cells which contain genes. Genes are grouped in thread-like structures called chromosomes.
Chromosomes are blocks of deoxyribonucleic acid (DNA) and contain detailed genetic instructions responsible for a wide range of factors, including:
- how the body’s cells develop
- the colour of the eyes
- the sex of a baby
Usually, cells contain 46 chromosomes. 23 are inherited from the mother and 23 from the father.
In people with Down syndrome, all or some of the cells in their bodies contain 47 chromosomes, as there is an extra copy of chromosome 21. The additional genetic material causes physical and developmental characteristics associated with Down syndrome.
Types of Down syndrome
There are three forms of Down syndrome, although the effects or each type are usually the same.
- Trisomy 21 is the most common type and affects around 94% of people with Down syndrome. Trisomy 21 occurs because every cell in the body has an extra copy of chromosome 21 (trisomy is a Greek word that means ‘third copy’).
- Translocation is caused when a piece of chromosome 21 attaches itself to another chromosome in a cell. It affects around 4% of people with Down syndrome. The symptoms of translocation Down syndrome are similar to those of Trisomy 21 but in rare cases it may be possible for a parent to pass translocation Down syndrome on to their child (see below).
- Mosaicism is the least common type, affecting around 2% of people with Down syndrome. Only some of the cells have an extra copy of chromosome 21. People with mosaic Down syndrome may experience less delay with some aspects of their development.
Some people do not have symptoms of translocation Down syndrome but they have altered genes that can trigger the condition in their children (translocation carriers).
The risk of a translocation carrier passing on the condition to their child depends on their sex (this is due to the way chromosomes are passed down through families). That is:
- male carriers have around a 1 in 35 chance of passing on the condition
- female carriers have around a 1 in 8 chance of passing on the condition
However, it is thought most cases of translocation Down syndrome are not inherited in this way.
The age of a woman when she becomes pregnant (the maternal age) is the only clear factor to be identified as increasing the chance of having a baby with Down syndrome.
For example, a woman who is:
- 25 years of age has a risk of 1 in 1,250
- 30 years of age has a risk of 1 in 1,000
- 35 years of age has a risk of 1 in 400
- 40 years of age has a risk of 1 in 100
- 45 years of age has a risk of 1 in 30
However, babies with Down syndrome are born to mothers of all ages and over half of babies with Down syndrome are born to mothers who are under 35 years of age. This is because women under 35 make up the majority of the child-bearing population.
It is not known exactly what causes the extra chromosome 21. It can come from either parent and there is no way of predicting the likelihood of a person making an egg or sperm with an extra chromosome. There is currently no clinical evidence that anything done before or during pregnancy causes the syndrome.
Last updated: 05 March 2013
Continue to next section: Diagnosing Down's syndrome