Our DNA is pretty cool. We get half our DNA from our mom and half from our dad and we function best when we have two copies of each chromosome. Having too much or too little of each section of DNA can lead to disease.
The most recognizable chromosome disorder is Down Syndrome. It results from having 3 copies of chromosome 21. There are only two other autosomes (not X or Y) that can survive – trisomy 13 and trisomy 18. That word ‘trisomy’ just means three copies of one chromosome – so trisomy 13 means 3 copies of chromosome 13 and two copies of the other chromosomes. Guess what monosomy means? Yep, one copy of the chromosome so monosomy 22 would be one copy of chromosome 22 with two copies of the other chromosomes. In case you hadn’t guessed, our chromosome complement is described as disomic = two copies.
We’ll go over the mechanisms in future posts (translocations and nondisjunction) but it is important to note that all chromosomes can be a trisomy. Trisomies 13, 18, and 21 aren’t more likely to become trisomic but they are probably better able to survive. Chromosome 21 is the smallest chromosome and may have fewer genes so having three copies may be less detrimental than a larger chromosome. It is interesting to note that the chromosome 1 is the largest and trisomy 1 has only been seen in very early miscarriages when mixed with cells with a normal complement.
Not only can you have extra chromosomes but it is also possible to have missing chromosomes = only one copy of a chromosome. In general, these are lethal. Only the X chromosome can survive as a monosomy – and 99.9% of these conceptions do not make it to term.
Just a note about the sex chromosomes: X and Y. First, X and Y are considered a pair so although they are two different chromosomes, you expect only XX (girl) or XY (boy) for a disomic (regular) chromosome complement. After saying that, there can be extra or missing copies of these chromosomes and it is probably due to the fact that there are dosage compensation mechanisms for these chromosomes. We’ll discuss X inactivation in future posts (the subject of my thesis so look out!) but basically, any X chromosome in excess of one is shut down so there is already a mechanism to deal with extra copies. Because extra Y chromosomes have had special attention in the media, we’ll talk about that separately but there is no evidence that having extra Y chromosomes is harmful.
Note: I haven’t discussed the manifestations of the chromosomal disorders because the students receive that information in lecture and it is well described in patient education materials. If there is time and interest, I will return to the topic with some clinical details.