Human chromosomes have a unique genetic code, which instruct and control the division, function as well as the growth of cells. Under normal circumstances, your cells contain 23 pairs of chromosomes, where one chromosome comes from your dad and the other from your mom. However, at times the cells could have three copies of a particular chromosome, instead of two. People who have three copies of the Chromosome 21 are born with Down syndrome. Other genetic problems occur when the duplicated chromosomes are other than Chromosome 21. Therefore Down syndrome is also commonly known as Trisomy 21.
Not a lot of people are aware of the fact that Down syndrome can be classified into three different kinds. Given below are facts about the three types of Down syndrome –
Standard Trisomy 21 or Nondisjunction Down syndrome
Around 90% to 95% of all the people who have Down syndrome are suffering from Standard Trisomy 21. Individuals who have 3 copies of Chromosome 21 in all the cells are born with this form of Down syndrome. Trisomy 21 occurs when there is an abnormal division of cells during the development of the egg cell or even the sperm cell. Around 88% of all Down syndrome instances come from the nondisjunction in the maternal gamete; while the paternal side constitutes of 8% of the total cases.
Standard Trisomy 21 occurs when there is a nondisjunction in the gametes before conception, which affects all the cells present in the child’s body. However, at times only certain cells are affected by the abnormal division, while others are just fine. This is known as Mosaic Down syndrome, or Mosaicism. There are two ways in which this form of Down syndrome can occur. The first is a nondisjunction process that occurs during the earlier stages when the cell divides in the embryo that is normal, which results in few of cells being affected by Trisomy 21. The other way occurs when an embryo with Down syndrome goes through nondisjunction, but certain cells within the embryo return to the standard chromosome arrangement. This form of the disorder is quite rare and constitutes for about 1% or 2% of observed Down syndrome instances.
Sometimes, Down syndrome is caused when part of Chromosome 21 becomes attached or trans-located to a different chromosome, before or during conception. People who have Translocation Down syndrome have the normal two copies of chromosome 21. However, they also have extra material from chromosome 21 attached onto the trans-located chromosome. Translocation Down syndrome is the only type that can be passed on from a parent to the child. This form of the disorder is also quite uncommon and constitutes for about 2% or 3% of observed Down syndrome instances.
- Facts about Down syndrome. National Institute of Child Health and Human Development. http://www.nichd.nih.gov/publications/pubs/downsyndrome.cfm. Accessed Feb. 14, 2011
- Genetic conditions: Down syndrome. Genetics Home Reference. http://ghr.nlm.nih.gov/condition=downsyndrome. Accessed Feb. 14, 2011
- Birth defects: Down syndrome. Centers for Disease Control and Prevention. http://www.cdc.gov/ncbddd/birthdefects/DownSyndrome.htm. Accessed Feb. 15, 2011