After Daniel was born, Brian and I were curious about how exactly Down syndrome occurs. So! Today you get a biology lesson! (Don’t leave, it’s interesting, for real! Okay, maybe we have different definitions of “interesting” in which case, see you tomorrow.)
For simplicity’s sake, I’m only going to go over how it happens when the egg provides the extra chromosome. This is true for something like 90% of cases. Also, this only explains cases of non-disjunction Down syndrome. This is the type Daniel has and roughly 95% of all cases of Down syndrome are non-disjunction.
Anyway, you may have heard at some point in your life that women are born with all their eggs. This isn’t *exactly* true. Women are born with all the cells that will turn into eggs, but they aren’t matured yet. Each group of cells ready to form an egg is called a primary oocyte. These primary oocytes each contain 46 chromosomes like any other cell in the female body. As ovulation approaches the primary oocyte divides into two new cells: a secondary oocyte and a small polar body. The secondary oocyte is what will continue maturing and become an egg. The polar body dies and disintegrates. The primary oocyte SHOULD split the chromosomes evenly, giving the secondary oocyte and the polar body 23 chromosomes each. In cases of Down syndrome the division is uneven, leaving both copies of chromosome 21 with the secondary oocyte. Roughly 80% of the time the error in cell division occurs here.
The secondary oocyte then goes through further maturation, dividing again into 2 cells, this time the mature egg and another polar body. Now the division reproduces the chromosomes (remember that a normal secondary oocyte will have 23 chromosomes) instead of dividing them, but there is a possibility that the mature egg with end up with an extra copy of chromosome 21 due to an error during this division. (If the secondary oocyte already has the extra chromosome it will reproduce here and be present in the mature egg.) About 20% of the time the error happens in this stage.
SO. There you have it. How (most cases of) Down syndrome happen. Exciting huh?