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Causes of Down's syndrome
We inherit genes from our parents and they determine our characteristics. Our genetic code is found in parts of our cells called chromosomes. Down’s Syndrome is caused by an extra number 21 chromosome. It can come from either the mother or the father and there is no way to predict it.
The genes that the extra number 21 chromosome contains cause extra proteins to be made, and this disturbs normal growth. Cells do not divide as rapidly as normal and this leads to a smaller than average baby. As the baby develops, cells do not move around in the usual way, which means that the body parts do not form normally, especially the brain. This means that the baby will have fewer brain cells and a different brain formation.
The likelihood of having a baby with Down’s Syndrome increases with the age of the mother, and is higher after the age of 35. However, more babies with Down’s Syndrome are born to younger mothers, because overall more babies are born to women under 35.
There are three forms of Down’s Syndrome:
Trisomy 21 – 94% of people with Down’s Syndrome have an extra chromosome 21 in every cell of their body. The two number 21 chromosomes do not separate but stay together in one cell (this is called non-disjunction). This type of Down’s Syndrome is often linked to the age of the mother.
Translocation – In 4% of cases, Down’s Syndrome is due to an extra part of chromosome 21 (rather than a whole extra chromosome). It occurs when small bits two chromosomes break off and stick together. Parental age is not a factor. In one third of these children a parent is found to be a ‘carrier’ of Down’s Syndrome and so has a higher than normal chance of having a child with the syndrome. But in two thirds the occurrence of Down’s Syndrome is random.
Mosaicism – In 2% of cases, there is an extra whole 21 chromosome in only some of the body cells and the rest of the cells are normal. These children are less noticeably affected and have less obvious physical characteristics. They may have little or no intellectual difficulties.
