What is “Down syndrome”?

trisomy 21October is national Down syndrome awareness month. So, what is “Down syndrome”?

Down syndrome is a genetic condition most often caused by a triplicate of the 21st Chromosome, also called Trisomy 21. Us “46-ers,” as I refer to those of us commonly called “typical” or “normal,” have 46 chromosomes. An individual with Trisomy 21 has an extra copy of the 21st Chromosome, so they have 47 chromosomes in their cells.

There are also two other forms of Down syndrome.

The first is called “translocation.” Individuals with this form of Down syndrome have the extra 21st Chromosome translocated to being attached to another chromosome, often Chromosome 14, but it can attach to Chromosome 13, 15, or another chromosome. Individuals with translocation Down syndrome physically cannot be distinguished from individuals with Trisomy 21. While the cause of Trisomy 21 is still unknown, translocation Down syndrome can be an inherited condition where one of the parents has one of their 21st chromosomes translocated to another of their chromosomes. In this case, the parent is not a “46-er” but a “45-er.”

Keeping with the theme of three being the magic number for Down syndrome (3 copies of the 21st Chromosome) the third “type” of Down syndrome is called “mosaicism.” With mosaic Down syndrome, instead of Trisomy 21 being present in every cell, only a certain percentage of cells has the extra 21st Chromosome. As the cells of the fetus divide, at some point one level of division divides with an extra 21st Chromosome. The extra copy is not present in all the cells, but it is present in a percentage of the cells of the person with mosaic Down syndrome.

Individuals with mosaic Down syndrome can go for some time not being diagnosed. I have been told that some individuals can be into their adult years when some percentage of mosaicism is diagnosed.

At a presentation in Columbus, Ohio, Dr. Alberto Costa, a researcher whose daughter has Trisomy 21, explained how cells in the brains of typical individuals can have various chromosomal counts including Trisomy 21. As he put it:

We may all have a little bit of Down syndrome in us.

And that seems fitting for all of us to keep in mind during National Down Syndrome Awareness Month.


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  2. […] as we are just $1,200 short of our goal. Feel free to donate in increments of $21 to signify the extra 21st Chromosome that is the cause of Down syndrome. Secure on-line donations can be made at this […]

  3. […] is National Down Syndrome Awareness Month. In previous posts, I’ve discussed what is Down syndrome and why the condition is called “Down syndrome.” But why is it a […]

  4. […] having those other attendant health concerns. She “just” has Down syndrome, an extra 21st Chromosome in every one of her cells. And, nothing has changed about the genetic condition. A child born with […]

  5. […] seem all the more typical. Everyone is there: those with just 46 chromosomes and those with the extra 21st Chromosome, mingling, talking, having a drink, and dancing at the annual awards […]

  6. […] commented on how the extra 21st Chromosome must have transformed over the decades. How else to explain the life expectancy rising from under […]

  7. […] Mrs. Darwin, at age 48, had a one-in-twenty-seven chance of giving birth to a child with an extra 21st chromosome, the typical cause of Down syndrome. It appears that she may […]

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