Twenty three. That’s how many pairs of chromosomes each of us has. They are numbered from 1 to 22 with two sex chromosomes – two Xs if you are female and one X and one Y if you are male. One of each pair comes from each parent. This number of chromosomes provides a balance. Two copies of every chromosome and two copies of every gene.
(As an aside – if you’re thinking about this carefully, you’ll realize that you actually do not have two copies of every gene. If you’re male, then each gene on your X chromosome and Y chromosome only have one copy each. And if you’re female, one copy of your X chromosome in every cell becomes inactivated (the inactivated copy is called a Barr Body, in case you were wondering), so technically only the genes on the active copy are being made into proteins. However, this is NOT the point of this blog post!)
So what if instead of two copies of every chromosome, you have three copies of a chromosome? This is called a trisomy – “tri” meaning that there are “three” of one chromosome. Examples of this include Trisomy 21 (three copies of chromosome 21), which causes Down Syndrome and Trisomy 18 (three copies of chromosome 18), which causes Edwards Syndrome.
There are also cases when you may have only one copy of a chromosome or two copies of a chromosome (when you’re only suppose to have one copy). Great examples of this are when you have more (or less) copies of the sex chromosomes:
- Only one X chromosome causes Turner syndrome, where the girls are usually shorter than average, have delayed or absent puberty, and may be infertile.
- XXY causese Klinefelter Syndrome, and these boys are taller than average, may have delayed or absent puberty and are often infertile
- Triple X (XXX) and XYY Syndrome, on the other are found in girls (XXX) and boys (XYY) and may cause them to be taller than average, but generally have no birth defects and no fertility problems.
The two big questions you may be asking yourself now are:
- Why would extra copies (or too few copies) of a chromosome have effects on a person’s development or behavior?
- How does this happen, anyway?’
Well, as you know, the DNA on chromosomes codes for proteins and these proteins have functions in the cells. If you have three copies of a gene, you will likely make 50% more protein. In the case of Down Syndrome, you will make more protein for all 310 genes on chromosome 21 including some genes that are involved in neuronal function.
As for how this happens…we haven’t talked a lot about how babies are made. I won’t get into how my Mom described it (though it does involve love and laying down very, very close to a man), but scientifically speaking, the egg and the sperm each have one copy of each chromosome (22 chromosomes plus one sex chromosome). These fuse during fertilization to create a fertilized egg (called a zygote) with all 22 pairs of chromosomes plus the two sex chromosomes. If, by accident, an egg or a sperm ends up with an EXTRA chromosome (though a process called nondisjunction, in case you were wondering), when it fuses to create the zygote, the zygote will have 23 pairs of chromosomes plus one extra.
Why don’t we have people walking around with all different combinations of different numbers of chromosomes? Mostly it’s because in most cases the zygote doesn’t survive if you add or take away chromosomes. The cells of the zygote do not divide or develop properly to create a human. It’s only in the cases where the zygote can divide and create a fully developed person (albeit a person who may have developmental or physical changes), that people with different numbers of chromosomes exist.