Todays installment of Reproduction Wednesday is about having too many chromosomes. The scientific term for this is aneuploidy, and the pirate term for this is “thar be too many etchings on yer peg leg!” Peg leg being a euphemism, in this case, for genetic material.
For the scientists among us – aneuploidy usually arises from failure of all the chromosomes to adhere to the mitotic spindle, causing one daughter cell to lose one copy of a chromosome, and the other to gain an extra copy of a chromosome.
For the non scientists – before yer cells split to form two new cells, they double their DNA and keep each chromosome joined together in the middle so as to keep ’em together. Then all the chromosomes line up in the middle of the cell, and a rope from each end of the cell (for lack of a better nautical term) attaches to each chromosome, so as to break them apart from where they are joined. If this messes up and they miss a chromosome duplicate, then one cell will end up with too many chromosomes and one too few. When it happens correctly, it looks like this in pictorial form!
This can happen either in the egg or the sperm, or in the embryo when it is only a few cells old. When it does happen it is generally VERY BAD. There are a lot of checks that an embryo has to pass before it can be brought to term, and generally having too many chromosomes means a fail, and it will not implant or it will miscarry early in the pregnancy.
There are a couple of exceptions to the you-fail-you-die-now dogma, the most common of which is Down’s Syndrome. People with Down’s Syndrome have three copies of chromosome 21 (5% of the time they have two copies of chromosome 21 and a little bit extra chromosome 21 attached to another chromosome. Isn’t genetics weird!) Most of the time it is the egg that screwed up and provided the wrong number of chromosomes. Interestingly (or perhaps, terrifyingly depending on your outlook) the chances of having a baby with Down’s Syndrome increases with the age of the mother, at age 20 it is 1 in 1500, but at age 40 it is more like 1 in 60. Ouch.
Other syndromes that arise from this kind of genetic miscalculation include Edwards Syndrome (three copies of number 18) and Patau Syndrome (three copies of number 13). These embryo’s rarely come to term, and only 10% of those that do survive past the first year.
Aneuploidy is not as bad when it concerns the sex chromosomes, good old X and Y. Having three copies of X doesn’t seem to make any difference to a woman. This could be because only one X is active in a female, the other one (or two, if you’ve got it) is wound up tightly in an inactive form called, piratey enough, a Barr Body. Similarly a boy with two Y’s an X is generally just the same as normal. XXY is not so great for a guy, as they usually end up sterile.
This stuff (aneuploidy I mean) happens in cancer too, an excessive amount! But that is a story for another time… Now I must away to see Up at the movies with Sexy Man!
Further info on aneuploidy can be found at this site, I heartily recommend it!