Down Syndrome

Down syndrome is a genetic disorder in which a person is born with an extra copy of chromosome 21. There are three genetic variations that cause Down syndrome: Trisomy 21, Mosaic Trisomy 21 or Translocation Trisomy 21. There are many ways in which theses disorders affect the body.

Trisomy 21 occurs when an egg or sperm comes in with an extra copy of chromosome 21, then, once an embryo is formed and starts to develop, the chromosome is replicated in every single cell of the embryo. Trisomy 21 is the most common type of Down syndrome. About 92% of Down syndrome patients have this type. People with Trisomy usually have physical problems.

Mosaic Trisomy 21 happens when an egg or sperm come in with an extra copy of chromosome 21, then, once an embryo is formed and starts to develop, the chromosome is replicated in some of the cells. In Mosaic some cells have a normal number of chromosomes and some have an extra chromosome 21. Mosaic Trisomy 21 occurs in 2-3% of patients with Down syndrome. Mosaic Trisomy produces a wide range of possibility of a person having physical problems.

Translocation Trisomy occurs when the extra copy of chromosome 21 or a piece of chromosome 21 becomes attached to another chromosome. Usually the chromosome 21 attaches to chromosome 14. About 3-4% of patients with Down syndrome have Translocation Trisomy 21.

Anyone can be born with Down syndrome because it is a random event. Down syndrome is not usually inherited, but can be inherited if one of the parents is a carrier. There is a greater chance of a child having Down syndrome if the mother is a carrier for the disorder, rather than the father. Also, there is a greater risk of a child having Down syndrome if the mother is over 35 years old.

There are three ways that a mother can be tested during pregnancy to see if their child will have Down syndrome. One is amniocentesis, which is the removal and analysis of a small sample of fetal cells from the amniotic fluid. Amniocentesis can not be done until the 14-18th week of pregnancy and with this process there is a lower risk of miscarriage than with the other two processes. Another process is chorionic villus sampling (CVS), which is the extraction of a tiny amount of fetal tissue at 9 to 11 weeks of pregnancy. The tissue is then tested for the presence of extra material from chromosome 21. Chorionic villus sampling has a 1-2% chance of the mother having a miscarriage. The last process is percutaneous umbilical blood sampling (PUBS), which is the most accurate method used to confirm the results of CVS or amniocentesis. During PUBS the tissue is tested for the presence of extra material from chromosome 21. PUBS cannot be done until the 18-22nd week and it carries a high risk of the mother having a miscarriage.

A person that has Down syndrome may have some physical problems or disabilities. Some common physical problems are: short necks, poor muscle tone, a small head and an overall smaller body.