The Three Types of Down Syndrome

Comparing Trisomy, Translocation, and Mosaic Forms of the Condition

chromosomes, human karyotype, down's syndrome, trisomy 21
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You probably know that Down syndrome is a genetic condition that causes a wide range of distinct physical characteristics and developmental delays. After all, it's fairly common: The National Down Syndrome Society (NDSS) states on its website that approximately one in 700 babies is born with the condition.

But did you know there are different types of Down syndrome, all of which involve the 21st pair of chromosomes?

The most common type, trisomy 21, is responsible for 95 percent of Down syndrome cases. Around four percent are called translocation Down syndrome, and the remaining one percent, mosaicism. Here's a brief look at how the three types of Down syndrome compare.

Trisomy 21 Down Syndrome

Normally human beings inherit 23 pairs of chromosomes from both the mother and the father for a total of 46. However, people with trisomy Down syndrome wind up with 47 chromosomes because they get an extra copy of chromosome 21. This happens when the 21st pair of chromosomes from either the egg or sperm fail to separate.

Translocation Down Syndrome 

Translocation Down syndrome occurs when two chromosomes, one of which is a number 21, join together at their ends. Instead of having three independent, separate number 21 chromosomes, a person with translocation Down syndrome has two independent number 21 chromosomes as well as a number 21 chromosome attached to another chromosome.

The connected chromosomes are called derivative chromosomes. Sometimes a derivative chromosome is inherited from a parent, but it also can happen for the first time in the person with Down syndrome. (This is known as de novo translocation). When a baby is diagnosed with translocation Down syndrome, it's important that her parents have karyotype testing to see if one of them carries the translocation, since it could be passed along to another child.

Although translocation Down syndrome occurs via a different mechanism than trisomy 21, the physical and features that mark the condition are the same.

Mosaic Down Syndrome

In mosaic Down syndrome, not all cells in the body have 46 chromosomes. Instead, a percentage of cells can have 47 chromosomes, with an extra copy of chromosome 21.

For example, in a sample of 20 blood cells from a baby with mosaic Down syndrome, 10 cells may have an extra chromosome 21 (47 chromosomes total) while the other 10 of these cells don't. Based on these counts, the baby would have a mosaicism of 50 percent. If this blood sample were representative of the blood as a whole, then half the cells in this child's blood would be normal, and the other half would contain an extra chromosome.

A person's level of mosaicism differs based on the sample of tissue take. For instance, a sample of skin could show a different level of mosaicism from a sample of brain tissue from the same person. In any case, people with mosaicism often have less pronounced physical traits and other characteristics, although as with anyone, it's impossible to make broad generalizations.