Down's Syndrome: Causes
1. Trisomy 21
This is the most common cause of Down’s syndrome. The most important part of conception and zygote formation is the mixing of the genes that takes place as one set of chromosomes arrive from the father’s sperm and the other set arrives from the mother’s egg. However, in some cases, the 21st chromosome from the mother arrives as a set instead of a single entity. Thus the embryo inherits an extra chromosome and the 21st set consists of three instead of two chromosomes. The genes on the extra chromosome 21 cause Down’s syndrome.
2. Mosaicing of cells
Two to four percent of the Down’s syndrome cases occur due to a phenomenon called mosaicing. Just like trisomy 21, the individual inherits an extra 21st chromosome. However, this inheritance is not uniform through all the cells. Some cells are normal while some are abnormal. Thus, while the individual may have perfectly normal blood cells, the muscle cells alone may contain a triad instead of a duet of chromosomes 21. Such individuals do not express all the symptoms of the condition and it can be so mild that it may even go undetected.
3. Translocation of chromosomes
Another 2 to 4 percent of individuals suffering from Down’s syndrome do not inherit an entire extra chromosome 21. A few genes from another chromosome (it is usually chromosome 14) dislocate to chromosome 21. This process is called translocation. In most of the cases, translocations are random events that cannot be determined or predicted. When such translocations occur in the individual, the extra genes on chromosome 21 exhibits as Down’s syndrome. Again, just like in the case of mosaic Down’s syndrome, this condition caused by the translocation of genes may not exhibit as intensely as in trisomy 21.
4. Delivery at advanced maternal age
Trisomy occurs due to faulty germ cell division in either the mother or father. However, most cases are due to the faulty division of the egg rather than the sperm. There is a very strong association between a trisomy in the child and the age of the mother. Thus, a mother conceiving at an advanced age stands a greater chance of having a baby with Down’s syndrome. The likelihood of a woman under 30 years of age giving birth to a baby with Down’s syndrome is less than one in a thousand. Note however that advanced maternal age is strictly not a cause of Down’s syndrome. It is a risk factor meaning that it increases the chances of the syndrome rather than directly cause it.
5. Being a second child after one with Down's syndrome
Another major risk factor is to have a brother or sister suffering from Down’s syndrome. Being the second or third child of parents who already have a child suffering from this condition increases the chances of getting this syndrome. This happens because the parents who have produced an anomaly in the genetic constitution of either the egg or sperm may produce the same anomaly again. Once again, this is a risk factor and not exactly a cause of Down’s syndrome.
6. Being a child of parents with translocation Down's syndrome
In some instances, a parent becomes a balanced carrier of the genetic translocation leading to Down’s syndrome. This means that the parent has exactly two copies of chromosome 21, but then, some of the genes belonging to this chromosome get distributed to another chromosome. Now, if the baby of such a parent inherits that chromosome with the extra genes which come from the chromosome 21, then the child will suffer from Down’s syndrome (i.e. two of chromosome 21 plus the extra chromosome 21 genes which are attached to some other chromosome). Thus, having a parent(s) with a translocation defect can lead to Down’s syndrome.
7. Random and unpredictable genetic events
Finally, there can be genetic anomalies due to a host of unexplained causes or effects. Any such mutations or changes can lead to trisomy 21 or translocations. So, Down’s syndrome can happen without any apparent cause too.