Ok So what is with the connection between the older the mother, the more likely the child is to have an extra chromosome? (Or is it missing a chromosome? I dont know).
I dont get it. Why would that have anything to do with it? Is it toxins from the ‘heavy metals’ and all that which is causing it?
Your DNA is composed of 23 chromosome pairs. You get one half of each pair from each parent.
Down syndrome is caused when you accidentally get a third chromosome #21 (trisomy 21). You either get two chromosome 21s from your mother and one from your father, or two from your father and one from your mother. The vast majority of Down cases are caused by an extra chromosome coming from the mother.
The precise cause is not known, but what is known is that something about the aging process interferes with the normal cellular machinery that divides up DNA when forming gametes, and thus eggs with doubled chromosomes are more likely to show up in older women.
I believe it is the same with all cell division. As we get older, our bodies are not as good at it and make mistakes. The same goes with ovulation.
I am not a medical geneticist but I am married to one. I will check with her tonight and see what she says.
Or on edit: what Friedo says.
Well…
Never mind, friedo said it better. 
Not to toss a monkey wrench in here, but it also appears that the rate among very young mothers is high, too (ages 15 years and less: the rates seem to be equivalent to those found at 30 or 35 years).
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My understanding is that human females are born with all the eggs they’ll ever have, and do not continue to produce them. Since the division of chromosomes is done already, how does age play a role?
Implied but not stated outright by the above is that all of a woman’s ova are as old as she is, whereas a man’s sperm get created anew throughout his productive lifespan.
Females are born with a full compliment of primary oocytes in their ovaries. The chromosome split does not happen until ovulation; at that point you finally have a mature ovum which is ready to be fertilized.
No, although it is often stated this way it is the oogonia that are present at birth. The actual ova (egg cells) are produced by later cell divisions by the oogonia.
BTW, oogonia is a great Scrabble word.
Oogonia. I think I visited there once.
Kindof humid. Warm, tho. Great sunsets.
Ahem, back to the OP - the older the eggs, the more likely there are to be mistakes in the coding. Mistakes in the coding = birth defects, chromosomal disorders (I don’t think I’ve ever actually spelt chromosomal before…) and other nasty things.
Also why older women have trouble actually getting preggers in the first place, carrying said pregs to term, and a whole host of other fun difficulties.
Nature did not have grad school in mind when we were developed.
Oddly enough, now that sperm/egg testing and IVF have gotten more precise, it’s been determined that older MEN also have more abnormalites with their sperm, so it’s not all on the ladies anymore.
Sweet! Finally, a topic about which I can pretty confidently state that I probably know more than anyone else on the boards. I was tangentially involved in doing some (at this point very preliminary) research on this very subject.
As it happens, human female oocytes are arrested in metaphase of meiosis I. At that point, the homologous chromosomes are paired up. They stay paired up until ovulation, so they can stay in this stage for decades. It’s long been known that this pairing is required for faithful segregation of chromosomes into daughter cells - that is, if they don’t stay paired, you might get both chromosomes going into the same cell, which (if it’s the 23rd chromosome that screws up and that egg gets fertilized) would cause Down syndrome.
So what happens to this pairing over time? Well, there are proteins responsible for keeping the two chromosomes together (called cohesins). Think of them as staples that are holding them tightly connected. It’s been assumed for years that those proteins get loaded on and then just stay there, presumably for decades, until the oocyte is activated and the chromosomes split. Unfortunately, this was based on work with yeast, which aren’t known for their long life spans. The fact is, it’s highly doubtful that ANY individual molecules can reliably stay in place for that long, so research is now going on to discover whether there’s a system in place that maintains the cohesion during the arrest period, either by replacing old cohesins as they wear out or by some other mechanism.
An experiment I did provided some of the first direct evidence that such a system probably does in fact exist, at least in fruit flies. If it holds up and if it turns out something similar is going on in humans (there’s a version of the key fruit fly gene in the human genome), then presumably the human system is beginning to break down as women age, leading to less faithful cohesion, and higher rates of nondisjunction and therefore Down syndrome. We’ll see.