Edwards syndrome (Trisomy 18)

Edwards syndrome, named after the British geneticist, John Edwards, who first identified it, is a chromosomal disorder where a person inherits an extra copy of chromosome 18 or a part of it. So instead of having two, they have three chromosomes 18 and so Edwards syndrome is also known as trisomy 18, in other words, “three chromosome 18s”.

All right, our DNA is like this humongous blueprint of information on how to make a human. Usually this massive document is packaged up nicely into a storage bin called a chromosome. Usually we have 46 chromosomes that we use to neatly organize all our information, depending on how you define organize. Each of the 46 chromosomes is actually part of a pair of chromosomes, since you get one from each parent, so 23 pairs. If you wanted to make another human, first you’d have to find someone that feels the same way, and then you both contribute half of your chromosomes, so one from each pair, right? Fifty-fifty. Now, what if someone contributes one too many? Say Dad contributes 23 and Mom contributes 24, is that possible? Yes, and it’s the basis of trisomies, in other words, having three copies of a particular chromosome. Now the most common trisomy in live births is trisomy 21, or Down syndrome. But the second most common trisomy in live births is trisomy 18, or Edwards syndrome.

Alright, so in order to package up half the chromosomes into either a sperm cell or an egg cell, you actually start with a single cell that has 46 chromosomes, let’s just say we’re making an egg cell for the mother, I’m just going to show one pair of chromosomes, but remember that all 23 pairs do this. So the process of meiosis starts, which is what produces our sex cells, and the chromosomes replicate, and so now they’re sort of shaped like an ‘X’—even though there are two copies of DNA here, we still say it’s one chromosome since they’re hooked together in the middle by this thing called a centromere. OK then the cell splits in two, and pulls apart the paired chromosomes, so in each of these cells you’ve now got 23 chromosomes. Now the two copies of the chromosome get pulled apart, and the cells split again, which means four cells, each still with 23 chromosomes. Now these are ready to pair up with a sperm cell from dad that has 23 chromosomes as well, totaling to 46 chromosomes, and voila–nine months down the road you’ve got yourself a baby.

OK let’s see how someone can end up with three copies of chromosome 18, instead of two. Well with Edwards syndrome, or trisomy 18, a process called nondisjunction accounts for most of the cases. Non-disjunction means the chromosomes don’t split apart. If the chromosomes in this first step don’t split apart, then one cell ends up with both chromosomes and the other gets none. Then the final result is 2 cells with an extra chromosome, and two cells missing a chromosome. Nondisjunction can also happen in the second step though, so first steps goes great, and both cells have a chromosome, but if they don’t split apart in the second step, then the final result is one cell with an extra chromosome, one cell missing a chromosome, and two with the right number of chromosomes. Now, if a sperm cell combines with any of these that have a duplicate of chromosome 18, then the combined cell will have three chromosome 18s, or trisomy 18.