Down Syndrome (trisomy 21, trisomy G) is a genetic disorder caused by the presence of an extra 21st chromosome.
The illness occurs in 1 of 800 children and causes the mental disorders and developmental problems. Related problems could vary from mild to serious ones. Mild condition is treated and such children and adults have more functional life.
Normal human cells have 23 pairs of chromosomes. Half of them come from mother and the other half from father.
The cause of Down syndrome is located on chromosome number 21. Added genetic material on 21st chromosome leads to development of this syndrome. This illness is caused strictly by genetic changes. Environmental or behavioral factor cannot cause it.
Genetic Variations that Lead to Down Syndrome:
- Trisomy 21 is causing more than 90% of all cases of Down syndrome. The name stands for three copies, triplet instead of pair of chromosome 21. Abnormal division of the sperm or egg cell during their development is causing this genetic disorder.
- Cases of mosaic Down syndrome are rare. Abnormal cell division after the fertilization is responsible for the change of the 21st chromosome.
- Translocation Down syndrome is a rare condition, too. There are two copies of 21 chromosome but also some genetic material from that chromosome is left on the translocated chromosome. This kind of chromosome changes happens around conception time.
In most cases, the Down syndrome is not hereditary, excluding some cases of translocation Down syndrome. It happens rarely, and usually one parent has rearranged genetic material and has no symptoms of Down syndrome, but could pass the translocation to his/hers children. This risk of passing the translocation is sex dependable. If the father of the child is a balanced carrier the risk is significantly low, there are just 3% chance that the child will have the translocation. But, if mother is carrying translocation, the baby has up to 12% chance to inherit the condition.
There are certain risk factors associated with the parents, which might lead to Down syndrome.
Women have greater chance to give birth to a child with a Down syndrome after the age of 45. The risk in pregnant women younger than 35 is 1 in 400, but rapidly increases to 1 in 35 cases in women after 45. This happens because older eggs (from older moms) are more likely to be improperly divisioned.
Lower possibility (about 1%) to have a child with Down syndrome have parents that already have a child affected with this condition.
Parents carrying the genetic translocation are at risk to pass the translocation for the Down syndrome to their child.