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There are fifty trillion cells in a human body, give or take. Each cell contains what’s called a nucleus, and inside each nucleus is the genome sequence—all of the genetic material that makes you who you are. A typical genome sequence is comprised of twenty-three pairs of chromosomes.

            You get twenty-three chromosomes from your mother and twenty-three from your father, for a total of forty-six. During conception, when the sperm fertilizes the egg, the mother’s twenty-three chromosomes come together with the father’s twenty-three, mix it up, and then begin dividing into new cells. Each cell contains a matched set, forty-six chromosomes.

            Unless something goes wrong. Sometimes chromosomes refuse to split, and you end up with an extra one in a cell—forty-seven chromosomes. That’s called nondisjunction. If it happens with any chromosome other than the twenty-first—which is the smallest—the cell development only rarely results in a live birth.

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           Here’s why chromosomes are so important: They include our DNA, the genetic code that makes us who we are. Chromosomes are microscopic, but they are densely packed with genetic material, that twisty rope thing called a double helix that you see in videos and textbooks. Unravel a chromosome and it can contain more than three meters of genetic material.

            That is what wreaks havoc with the development of a person with Down syndrome, also called Trisomy 21, which stands for third twenty-first chromosome. The twenty-first chromosome is the smallest, but it still includes enough genetic material to make a dramatic impact if you have an extra one in each of your trillions of cells. (Or even if you have an extra twenty-first chromosome in selected cells—that’s called mosaicism, and is a less common, often milder form of Down syndrome.)

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           Scientists have only recently begun unwinding our chromosomes to find out what the genes contained in them do. For a long time, we knew that there was an increased chance a person with Down syndrome would get leukemia and early-onset Alzheimer’s disease, and a decreased chance that the person would get tumor-based cancers. Now scientists can point to the genetic material in the twenty-first chromosome and begin to say why.


When Sophie was 10, a scientist friend volunteered to have karyotypes made, so she and I could see our chromosomes side-by-side. Sophie's (right) includes the extra 21st chromosome. She had been hoping a picture would come back of our chromosomes cuddling. 


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