General Information
Definition
Down Syndrome is a chromosomal condition caused by the presence of all or part of a third copy of chromosome twenty-one. The chromosomal abnormality results from a genetic mutation known as nondisjunction. DS is the most common chromosomal abnormality in humans and is typically associated with a delay in cognitive development and physical growth, along with distinctive facial characteristics. In most cases, DS is diagnosed after chromosome tests have been administered shortly after birth.
Variations within the Condition
There is a wide variation in mental abilities, behavior, and developmental progress in individuals with Down syndrome. Their level of intellectual disability may range from mild to severe, with the majority functioning in the mild to moderate range.
Three Types of DS: Trisomy 21, Translocation DS, and Mosaic DS
Three Types of DS: Trisomy 21, Translocation DS, and Mosaic DS
- Down syndrome (or Trisomy 21) accounts for ninety-five percent of people with Down syndrome. A child with Trisomy 21 has three copies of chromosome 21, rather than the normal pair.
- Translocation Down syndrome accounts for just three to four percent of people with Down syndrome. Translocation is what people are referring to if they say that the condition is inherited, because usually one parent is a carrier. The extra #21 chromosome is present, but attached to a different chromosome in the egg or sperm. The clinical features of people with Translocation Down syndrome are indistinguishable from those with Trisomy 21.
- Mosaic Down syndrome accounts for less than one percent of all people with Down syndrome. Children born with Mosaic Down syndrome have some cells with three copies of chromosome 21 and some cells that have the usual pair. Clinically, babies born with Mosaic Down syndrome can have the same features and health problems seen in babies born with Trisomy 21 or Translocation Down syndrome. However, the presence of cells with the normal number of chromosomes may result in fewer characteristics of Down syndrome.
Incidence
Approximately 5,000 babies are born with DS each year. 1 in every 733 babies are born with the condition, and 80% are born to women under the age of thirty-five. The life expectancy of people with DS is approximately sixty years old.
Sources:
http://www.mayoclinic.com/health/down-syndrome/DS00182/DSECTION=causes
http://www.nads.org/pages_new/facts.html
http://www.globaldownsyndrome.org/about-down-syndrome/history-of-down-syndrome/?page_id=4578
Sources:
http://www.mayoclinic.com/health/down-syndrome/DS00182/DSECTION=causes
http://www.nads.org/pages_new/facts.html
http://www.globaldownsyndrome.org/about-down-syndrome/history-of-down-syndrome/?page_id=4578