Skip to content
My WebMD Sign In, Sign Up

Children's Health

Font Size
A
A
A

Classification of Down Syndrome

The three basic types of Down syndrome are trisomy 21, translocation, and mosaicism. All types of Down syndrome result from irregular chromosomes in some or all of the baby's cells. These irregularities are caused by abnormal cell division, usually in the egg but sometimes in the sperm, before or after conception. It is not completely understood why this occurs.

Trisomy 21

Most people who have Down syndrome have an entire extra chromosome (47 instead of 46) in every cell in their body. This type of Down syndrome is not genetically inherited. Abnormal cell division occurs in the egg (95% of trisomy 21 cases) or sperm (5% of trisomy 21 cases) before or after conception. As the embryo develops, the extra chromosome is copied (replicated) in every cell of the body.

Recommended Related to Children

Kids Need Real Happy Meals

This year, start a new tradition pack a healthy school lunch for your kids. A nutritious lunch from home helps kids stay away from the high-fat, high-sugar, and high-sodium stuff in school cafeterias and vending machines. Research shows that America's children are missing out on five essential nutrients to their growth and good health: calcium, fiber, magnesium, vitamin E, and potassium. An ideal lunch provides good nutrition plus just enough calories to fuel body and brain throughout the...

Read the Kids Need Real Happy Meals article > >

Older women are at higher risk than younger women of having a baby with trisomy 21. As a woman ages, the chromosomes in her eggs are more likely to divide incorrectly.

Translocation

A few people with Down syndrome have a type called translocation. This type of Down syndrome may be passed down through families (inherited), but most cases are chance events. Although the total number of chromosomes is normal (46), a part of chromosome 21 breaks off, attaches to another chromosome, and produces the signs and features of Down syndrome.

Mosaicism

A few people with Down syndrome have a type called mosaicism. This type is not inherited. Mosaicism results from abnormal cell division in only some cells after fertilization camera.gif, while others divide normally. The cells that divide abnormally produce 47 chromosomes, with the extra genetic material from chromosome 21. As the embryo grows, both normal and abnormal cells continually replicate. Even though half of the cells may be normal, symptoms usually are nearly the same as those that occur with other types of Down syndrome. But children with more normal than abnormal cells may have fewer features of Down syndrome.

By Healthwise Staff
Primary Medical Reviewer Susan C. Kim, MD - Pediatrics
Specialist Medical Reviewer Louis Pellegrino, MD - Developmental Pediatrics
Last Revised July 20, 2011

WebMD Medical Reference from Healthwise

Last Updated: July 20, 2011
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

Today on WebMD

preschool age girl sitting at desk
Article
look at my hand
Slideshow
 
woman with cleaning products
Slideshow
young boy with fever
Article
 

worried kid
fitArticle
boy on father's shoulder
Article
 
Child with red rash on cheeks
Slideshow
girl thinking
Article
 

babyapp
New
Child with adhd
Slideshow
 
rl with friends
fitSlideshow
Syringes and graph illustration
Tool
 
6-Week Challenges
Want to know more?
Build a Fitter Family Challenge – Get your crew motivated to move.
Feed Your Family Better Challenge - Tips and tricks to healthy up your diet.
Sleep Better Challenge - Snooze clues for the whole family.
I have read and agreed to WebMD's Privacy Policy.
Enter cell phone number
- -
Entering your cell phone number and pressing submit indicates you agree to receive text messages from WebMD related to this challenge. WebMD is utilizing a 3rd party vendor, CellTrust, to provide the messages. You can opt out at any time.
Standard text rates apply

WebMD Special Sections