The human body is made up of cells. Each cell is like a factory which has everything needed for growth and to maintain the body. Every cell contain a nucleus in which genes are stored and these genes are grouped in thread-like structures called chromosomes. Usually, the nucleus of each cell contains 23 pairs of chromosomes, half inherited from the mother and half from the father, making 46 chromosomes in all.
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. This additional genetic material results in the range of physical and developmental characteristics associated with Down syndrome.
Three types of genetic variation are relevant:
Trisonomy 21 – in which all the cells of the body have an extra chromosome 21.
About 94% of people with Down Syndrome have this type.
Translocation – in which extra chromosome 21 material is attached to another chromosome.
Around 4% of people with Down Syndrome have this type.
Mosaic – in which only some of the cells have extra chromosome 21 material.
About 2% of people with Down Syndrome have this type.
The type of genetic variation that children experience does not significantly alter the effect of Down syndrome. However, individuals with mosaic Down syndrome appear to experience less delay with some aspects of their development.
Down Syndrome affects people in all races, religions and economic backgrounds and in all countries around the world. It can happen to anyone. Although the chance of having a baby with Down Syndrome increases with the mothers age, babies with the syndrome are born to mothers of all ages. Over half of children born with Down syndrome are born to mothers under the age of 35 years.
As yet, no one knows what causes the presence of the extra chromosome 21. It can come from the mother or the father. There is no way of predicting whether a person is more or less likely to make an egg or sperm with an extra chromosome.
What we do know is that no one is to blame. As far as we know, nothing done before or during pregnancy causes the syndrome.