What are the Causes of Down Syndrome?
Cells of human normally contain 23 pairs of chromosomes, that means 46 in all. One of the chromosomes comes from father's sperm and other from the mother's egg. When the sperm and the egg cell join together they form a fertilized egg with 23 pairs of chromosomes. Down syndrome is caused due the abnormal cell division of chromosome 21. This abnormal cell division results in extra genetic material from chromosome 21, which is mainly responsible for causing Down syndrome.
There are three types of cell division that occur in the 21st chromosome. Down syndrome is caused due to one of these types of cell division. The types of abnormal cell divisions that cause Down syndrome include:
Trisomy 21 Down Syndrome
This is the most common form of Down syndrome and about 90 to 95 % of the cases of Down syndrome are caused due to Trisomy 21. In this case, Down syndrome is caused due to the abnormal cell division during the development of egg or sperm cell. An egg or sperm cell may divide incorrectly, which may further result in the fertilized egg having an extra chromosome 21. That means the fertilized egg has in all 47 chromosomes instead of 46. This type of Down syndrome is referred to as trisomy 21 because the fertilized egg has three copies of chromosome 21 instead of two.
Mosaic Down Syndrome
This is the rare form of Down syndrome and only about 1 to 2 % of the cases of Down syndrome are caused due to mosaic Down syndrome. In this case, Down syndrome is caused due to the abnormal cell division after fertilization. Children affected with this type of Down syndrome have some cells with an extra copy of chromosome 21 and others with the normal number. This type of Down syndrome is referred to as mosaic Down syndrome because it is a mixture of normal and Down syndrome cells.
Translocation Down Syndrome
This is also the uncommon form of Down syndrome and about 3 to 4 % of the cases of Down syndrome are caused due to translocation Down syndrome. In this form of Down syndrome a part of chromosome 21 breaks off and gets attached or translocated to another chromosome in the egg or sperm cell, before or during fertilization. The fertilized egg may have translocation Down syndrome. Children affected with this type of Down syndrome have the usual two copies of chromosome 21 plus additional chromosome 21 material attached to another chromosome.
Down syndrome is not usually hereditary. It is usually caused by a mistake in the cell division during the formation of reproductive cells. Only in case of balanced translocation Down syndrome, the Down syndrome can be passed from parent to child. In this case of Down syndrome, either a father or a mother is a balanced carrier of the translocation. This means that either of them has some rearranged chromosome material but no additional chromosome material. Only 4 % of the children are affected with translocation Down syndrome, and only about half of these cases are inherited.
Remember that Down syndrome is not caused by environmental factors or behavioral activity of parents. Increasing maternal age is also considered to be one of the causes of Down syndrome. It has been established by researchers that, as the women get older the chance of a reproductive cell containing an extra copy of chromosome 21 increase. Therefore an older mother has an increased chance of having a baby with Down syndrome than the younger mother.
About 25% of the babies affected with Down syndrome are born to the women of age 35 or above. Therefore many health care providers recommend prenatal testing for Down syndrome for the women who become pregnant at age 35 or older.