Sounds cool, Teppy. The alternative could just be "paintings."

Damn, how much do I love having a medical and science writer in my writers' group?

Having run my take on "level four pathogen before we grokked there were such things, only not contagious, triggering instead" scenario for "Burden of Memory" in his lap? He came back to me within five minutes with this:

Well, off the cuff, it could be a virus that enters only the germ line and intergrates itself there. I am not sure why it would just target these cells, though it would be a really nifty way to replicate itself from host to host (being a nonlethal expression, maybe it just integrates into all cells including, for some advanced reason, germ cells). Another idea, again, just thinking out loud, is to have infection similar to the ways HIV is passed onto a baby: mom has it and during delivery the baby gets exposed and thus infected. This would only pass it along maternally, however, though that could be interesting and gets you to your next generation.

Have now pinged back going, sounds amazing. What precisely is the "germ line"?

What precisely is the "germ line"?

There are two types of cells in the body: somatic cells and germ cells. Germ cells are the reproductive cells, the egg and sperm. If your virus entered the germ line, it could be passed down through generations as well as sexually. I think it needs to penetrate more than just that for it to have physical effects on the carrier, though that might be the catch: the carrier is just that, a carrier, and it's only the child that manifests the disease, having been created via the genetically modified germ cell. Also, one of the key differences between somatic cells and germ cells is that somatic cells divide by mitosis and germ cells divide by meiosis, in which the cell divides but halves the chromosome number before undergoing a round of regular mitosis, forming four haploid cells from one diploid cell. Oh, gametes. I knew there was another word I was more used to calling them.

Now, another idea would be for it to hit any sort of stem cell, like in the bone marrow, because if it modifies those, all the various cell types it differentiates into would be affected. And there are, in fact, hematopoietic stem cells, which would be perfect.

If I've just confused you more, the answer to your question is "sex cells."

Actually, this works nicely, because I don't want it transmitted sexually; I want it dormant (latent?) in the carrier, a la females with the messed up Factor 8 thing for classical haemophilia, but I want a percentage of the kids to have it.

Would it work that way?

I think if it entered the germ cells and only the germ cells, it would not affect the carrier (because those cells aren't differentiating into anything except sperm and/or egg), but a child created by an infected carrier would manifest your disease, as the stem cell would differentiate into the various organs of the body. That child would probably have the capability of passing it down to future generations as well. I'm not certain it wouldn't pass to someone through intercourse not resulting in pregnancy (I'm not totally clear on the transmission vectors of STDs), but I think you could say it didn't.

Again, the important thing about a sperm or egg cell is it has half the number of chromosomes (23) as in a normal cell (46). Well, that's one important thing, and it might be a way to work out specificity.

So, the percentages being that if one parent had the mutated chromosome, and had two kids, one kid has - what? 50% of manifesting it?

It depends on the nature of your virus. If it affects a specific chromosome in the parent, then yes, there's a fifty-percent chance of a child being infected: the sperm or egg is formed via meiosis, and half of the gametes will get one half of the chromosome pair. Think of sex-linked diseases: there's a fifty-percent chance of a man passing down an X or a Y.

I feel like your virus is wackier than that, and fundamentally fucks with the germ cell such that any child will manifest. Like, it could do a lot of genetic damage, so any of the gametes formed are guaranteed to be abnormal.

(furiously marking posts)

Deb, this page might be helpful in understanding genetic transmission of diseases, especially the more unconventional kind. Of special notice is the section on mitochondrial DNA, if you want your disease to be transmitted only from the mother. I'm going to sleep now, but if you have any more questions, I'll be here tomorrow.

'tis good. Sleep well!