What causes Down Syndrome?

Down Syndrome is caused by an excess of genetic material. The genetic material is present in every human body cell and is arranged in tiny structures called chromosomes. They are the building blocks which gives us our individual characteristics. They determine the colour of our hair, our eyes and many other characterisitics.

Each human body cell contains 23 pairs (i.e. 46) chromosomes, except the sex cells (egg cells of a woman and sperm of a man). which each contains only 23 chromosomes. Egg cells and sperm are formed by a special cell division process during which the chromosome

pairs divide and only one chromosome of each pair is included in a sex cell. The egg cells and sperm therefore each receives only one copy of each chromosome pair.

During fertilisation a sperm of the father fuses with an egg cell of the mother. The fertilised egg cell then again contains 23 pairs (or 46) chromosomes. One chromosome of each pair comes from each of the parents. The fertilised egg cell divides rapidly in the womb and eventually the baby is formed. Each of the body cells of the baby also contains 23 pairs (46) chromosomes (see Figure 1).

When a blood sample of a baby is analysed in a laboratory, a photograph of the chromosomes is taken under a microscope. The chromosomes on the photo are then cut out, numbered and arranged according to pairs. Such a chromosome arrangement is called a karyotype (see Figure 2). Note that the chromosomes are arranged and numbered .according to a specific pattern, namely from large to small. One of the smallest chromosomes is numbered as no. 21. It is this chromosome no. 21 that is involved in the origin of Down Syndrome.

There are different types of Down Syndrome, viz.

>b>The Tritomy 21 type of Down Syndrome

Trisomy 21 occurs when the two no. 21 chromosomes fail to separate and both instead of one, become incorporated into either the egg cell or the sperm. This cell then has 24 chromosomes instead of the normal 23. This phenomenon of the chromosome pair not separating is called noadisjunction.

With fertilisation, the sex cell of one of the parents (egg cell or sperm) with the two no. 21 chromosomes (and therefore 24 chromosomes in total), fuses with the normal sex cell of the other parent (egg cell or sperm with 23 chromosomes) to form a fertilised egg cell with 47 chromosomes. The baby that develops from this cell will therefore have an extra no. 21 chromosome in each body cell and hence Down Syndrome (See Figure 3).

The trisomy 21 type of Down Syndrome may be caused by non-disjunction during the formation of either the egg cells of the mother or the sperm of the father. The chances of non-disjunction however seem to be increased in women in their late thirties and older (see Figure 4).

The cause is unknown, it is important to understand that it is not caused by anything the mother or father did or failed to do during the pregnancy.

The trisomy type of Down Syndrome is the most common type of Down Syndrome and is the cause of approximately 90-95% of all cases of this syndrome. Trisomy Down Syndrome is not hereditaryl In young mothers the chance of a second baby being born with Down Syndrome is therefore very small.

The mosaic type of Down Syndrome

This type is very rare and is caused by non-disjunction of the chromosome pair no. 21 shortly after fertilisation. The result is that some cells contain an extra chromosome no. 21 (and therefore 47 chromosomes) while the other cells contain only 46 chromosomes (the normal number). This type of Down Syndrome is also not hereditary and the chances of a second baby born with it is small.

It is important to remember that the type of Down Syndrome that is diagnosed, makes no difference to the eventual development and potential of the child.

The translocation type of Down Syndrome

A baby sometimes has all the clinical symptoms of Down Syndrome but only 46 chromosomes in every cell. In these cases it is found that. over and above a normal pair no. 21, these babies also have an extra part of a chromosome no. 21, attached to another chromosome, which usually is chromosome no. 14. This is called the translocation type of Down Syndrome.

This type of Down Syndrome can be hereditaryl In this case the risk to have another child with this type of Down Syndrome is increased and it is strongly advised that a chromosome test is done on both parents and that they seek genetic counselling before planning further pregnancies.