AAAAAAGH! No, that's not *quite* right there.

OK, first off I shouldn't have read this thread because I wanted to look up why larger ladies had trouble TTC and all I could find was that "oooh, you're so thin and don't eat enough so you don't even ovulate" for me - which really upsets me, especially as it takes me months to put on any weight and has anyone tried finding full-fat food these days? I don't eat lo-fat through choice! - I'm so upset I even need to use an emoticon! And as for exercise - if I owned a car I wouldn't get anywhere near as much exercise! How did women of my grandparent's generation manage to have children? I guess they had full-fat stuff. I can't have much of that even if I wanted it because of an hereditary cholesterol problem.

Anyway, I do know that excess fat and cause pregnancy complications such as gestational diabetes, but nothing in particular about how excess weight affects TTCing - although we all know it does to some degree.

Now, for the genetics. You don't have a "daddy" ovary and a "mummy" ovary. What you have is one chromosome from dad and a corresponding chromosome from mum. Both of these are marked so we know from whence they came - there's in interesting thing with C16 - 2 daddy C16s and no mummy C16s gives you Prada-Willi Syndrome, 2 mummy C16s and no daddy C16s gives you Angelman's Syndrome. Really interesting - happens when there are 3 C16s in a newly-fertilised embryo and one is kicked out, but the imprinting still affects the baby. Also daddy genes make the placenta, mummy genes stop the placenta being so invasive and make baby grow as well as placenta. No daddy genes, no placenta; no mummy genes, no baby grows - it really is all placenta! Anyway, fascinating as this all is, the X chromosome works a bit differently. All the imprinting is wiped off in the ovary, so all your eggs are mummy-imprinted.

You still have a daddy X and a mummy X, and XX makes us girls and XY makes a boy. So how can boys do with just one X? That's because in every cell one X is inactive! It curls up into a little ball and just isn't used apart from when the cell divides. The inactivation is random and doesn't happen in boys (unless they're XXY). So that's why we can also have girls with one, three or four Xs (though they have seperate fertility issues). Genes such as colour-blindness are on the X, which is why it's more prevalent in boys than girls, although if your son is colour blind you're actually half colour blind: about half your active Xs will be colour blind, the other half not, so you don't appear to be colour blind (or only slightly colour blind). The X is fantastic!

So, what could be happening is that if your daddy X isn't doing the business, you'll only ovulate half the time, because half of the time that X will be the active X. While it is theoretically possible that one ovary could be only daddy-X active and the other only mummy-X active (really should use the words maternal and paternal, but too late for that now I guess) that's the equivalent of throwing a coin 10'000 times and getting a head every time.

Anyway, both ovaries have both sets of parental genes, just the active X-chromosome may different in the cells that stimulate the follicles. I don't even know if this is sex-linked, I'd just assume so because it's to do with fertility and also because of the stupid "mummy ovary" and "daddy ovary" comment.

Anyway, genetics lesson over! Any questions?