Trisomy 21

I don’t have a “today’s fact” because this whole post is information about Down syndrome. First, some basic science-y stuff.

People have 23 sets of chromosomes. Each pair is numbered, 1 through 23, with the 23rd set being the sex chromosomes. A third copy of any chromosome is called a trisomy, and is followed by the number of the chromosome with the third copy. Down syndrome, for instance, is Trisomy 21- a third copy of the 21st chromosome.

When the chromosomes were first numbered, they were numbered in order of how much genetic information they hold (with the exception of the sex chromosomes, which are at the end just because.) So the first chromosome is the biggest, and so on down the line. Here’s a picture of all the chromosome pairs:


{photo from here}

As you can see, each pair is smaller than the pair before. But look closely, and you may notice that chromosome 21 is actually smaller than 22. When the chromosomes were originally numbered they (scientists, or geneticists, or whoever does that stuff) thought that 22 was the smallest. It turned out later that wan’t true, but it was too late to re-number.

Anyway, the fact that chromosome 21 contains the least genetic information is the reason Down syndrome is the most common chromosome disorder. A third copy of the 21st chromosome has less impact than a third copy of any other chromosome. In theory a trisomy can occur with any chromosome, but in reality very few trisomies are compatible with life. Of the few that are seen in live births, Down syndrome has by far the highest survivability rate, with only about 25% of fetuses with Down syndrome being miscarried or stillborn. (I would say 75% surviving, but that doesn’t take into account termination rates in the case of prenatal diagnosis, and that’s a whole separate post.)

So there you have it- today’s genetics lesson. Any questions?


5 thoughts on “Trisomy 21

  1. I actually didn’t realize that Downs had that low of a survivability. I mean one out of four is pretty high. Obviously much better than the other trisomies but still much higher than I would have thought.

    • That was higher than I expected too. Although, it is somewhat of an estimate, since so many miscarriages happen too early for testing.

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