Mr. Rilch and I were discussing American Empire: Blood and Iron last night, focusing on the character of Jake Featherston. I said, “He’s such a total cracker. Like maybe one set of great-grandparents were cousins. I can just see him: one generation away from inbred.”
Mr. Rilch says, “If he’s one generation away, that would be one set of grandparents were cousins.”
I’m still not sure which of us is right. In my theory, Ann and Bill would be cousins, and marry to produce Clem. Clem would marry Dorothy, who would produced Emily, who would marry Fred and produce Jake. Clem would be inbred, and Emily wouldn’t, so Emily and Fred would be the one generation separating Jake from inbreeding.
Mr. Rilch says one generation away would mean Clem and Dorothy would be cousins. Therefore, Emily would be the product of inbreeding, but if Fred wasn’t related, they would be the “one generation”.
Did I explain that right? I’ve gone over it three times.
Anyway, beyond that, how far-reaching are the effects of inbreeding? Would the offspring of an inbred and a non-inbred be “pure”? Is there a particular fraction one has to reach, like 1/8 or 1/16th? Or is no one pure if they have even one anscestor who was inbred?
You can calculate something called an “inbreeding coefficent” fairly easily. To understand what this is, you have to understand why inbreeding is a concern.
Genetically, there’s nothing necessarily inherently bad about inbreeding. If there are no bad genes in the shared genome, there’s no problem. However, that’s not a realistic situation. In real life, we all have recessive genes that would be harmful if expressed. Think of it this way - we have two copies of every gene. If one copy is makes a broken protein, it’s probably not a big deal, because your other copy can often compensate. It’s only a problem if both your copies are broken.
Now, if I pick someone at random out of the public and breed with her (if only!!), the odds are very slim that her bad mutations match up with mine, so our kids wouldn’t have two bad copies of any particular gene. However, if I bred with my sister (eeeew!), she would have many of the same defects that I have, and the odds are pretty good that our kids would express some of these bad genes.
Whew. The inbreeding coefficient tells you exactly what the odds of these problems are. An inbreeding coefficient of .5 means that each child has a 50% chance of having a genetic problem caused by inbreeding. They could have other, unrelated problems, of course.
I don’t recall offhand how to calculate this, but it’s pretty simple. A google search, which I’m too lazy to do right now, would probably get you some good sites.
Quick follow up - I always think of one more thing I should have said when I hit “Submit”.
Going back to your example, the inbreeding coefficient would go up with each generation. I don’t recall offhand for sure whether or not it can ever hit 1, but I don’t think so.
Iceland…the population of Iceland is traced back to the 1000 or so who survived a plague about 500 (?) years ago.
most of the Icelandic women who have breast cancer caused by a faulty gene can trace their ancestry back to ONE man.
Iceland is used by most geneticists as a fertile study ground because of the lack of genetic diversity, and the good records kept as to the genealogy of the populace.
also, Northern Ireland has very high rates of Spina Bifida, Coeliac’s disease, coronary heart disease and Cystic Fibrosis. this is thought to be because of the inbreeding among the protestant settlers. a baby born in Belfast is about 4 times more likely to have CF than a child born in London.
in both cases we’re talking consequences of inbreeding carrying down through 5 or 6 generations.
Everyone of us has about five to ten lethal recessive genes. If you produce a child with a close relative, those recessive genes are more likely to be expressed in the offspring.