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!

Deb, you might check on how Albinism is passed down. It's a recessive that is sometimes passed down through the mother. There are a couple of different kinds, and if you modeled your disease on that, it could have a certain percentage of the children who get it die, a certain percentage do what you want, and a certain percentage where it's not really obvious that they have it at all. This page explains it for the layperson pretty well.

Deena, you're a mind-reader, because that was the next thing on my list. I'd been looking at something that would a) be viral or possibly bacterial in nature (remembering that one of my early theses was on the Black Death), b) not evince symptoms in the initial carrier, but c) trigger a chromosomal malfunction in said carrier's descendents.

The first thing that comes to mind is haemophilia (I can never remember whether it's factor 7 or factor 8 that's affected there), because the results were so damned visible, courtesy of the Tsarevitch and Rasputin. And most of the conventional wisdom when I was growing up was that it was through Victoria's line, her father was what, 57, when she was conceived, a mutation in the sperm.

But of course there are other conditions as well.

My neurologist is actually in a pool of highly research-oriented doctors who believe that MS is triggered virally.

Since the science in this one needs to be accurate - it's the lynchpin for the story, and the main reason the two older vampires have to work with their potential dinner, instead of eating it - research is indicated.

The grandparents affected are going to be Hollywood industry bankers and backers in the 1950s, big money types. I'm going to use William Holden's Mount Kenya Safari Club, or a slightly earlier version thereof, as the site where the viral species jump first happens over dinner. And Nic, bless him, sent me all the info about how the vampires can find instances in their own cultural lore about earlier outbreaks.