Blue Collar Scientist

Several times over the years I’ve tried to explain to various friends the concept of pedigree collapse. The idea is simple - as you go back in time, every generation of your ancestors has twice the number as the one before. You are one person; your parents are two people; you have four grandparents; eight great-grandparents, and so on. If you follow this back far enough, you eventually come up with a number of people in an ancestral generation that is greater than the number of people who were alive on the Earth at that time.

Of course, well before then, you will probably find a lot of your ancestors having more than one relationship to you. Imagine for a moment a couple who have two children, Jack and Jill. They get married to their respective spouses, and have children. Their children marry, and have children. This goes on for a dozen generations - which works out to three hundred years - and one of the descendants of Jack marries one of the descendants of Jill. Now you’ve got Jack and Jill at two different places in your ancestry - maybe Jill is both your greatⁿ grandparent on your father’s side, and also on your mother’s. Similar situation for Jack, and for Jack and Jill’s parents.

Now when you explain this to someone who understands math or biology, they say something like “yeah, what’s the big deal?” as though you’ve made an insight of no brilliance whatever and are rehashing something that is self-evident and obvious. They expect you to take it somewhere interesting, because by itself, this is old news and not really worth talking about.

But talk about this to other people, and it can be a weird experience. A very small minority of the educators that I’ve known and worked with have reacted with abhorrence to the concept of pedigree collapse. It’s strange, because these are generally special educators, people who - you would (incorrectly?) think - had been exposed to more population genetics than the average teacher as a result of their advanced degrees that qualify them to work with people having challenging conditions mediated by genetics and environment.

But in some cases at least, this expectation has been wrong, and these people have responded with a visceral abhorrence to pedigree collapse, branding it as “gross” and the person who brought it to their attention (i.e., me) as morbid and strange¹.

It’s a hypocritical reaction, obviously. These people are not going to demand that their fiancee provide 300, or 500, or 1,000 years of their genealogy to prove that they have no ancestors in common in distant genealogical time before getting married. They live their lives by one standard, but when it comes to talking about human issues, they want nothing to do with it.

My friend (and almost-certainly distant relative) Martin Rundkvist wrote a post a couple days ago about the subject of pedigree collapse, looking at it from the bright side - we’re all descended from royalty, and we’re all descended from St. Olof - a prediction you can safely make by the staggering numbers of ancestors we would all have at that time. Martin even offers a brilliant rejoinder to the criticism that human gene flow is limited by cultural structuring - the idea that classes do not sexually mix, and that gene flow between race and even nation-state is hindered:

On the contrary! Nobody knocks up more serving girls than the king’s younger brothers.

It sounds plausible to me. I’ve wanted to - well, not exactly knock up, but do the deed with a few serving girls myself.

1. So much for the supposedly non-judgemental attitudes of people who work with special-ed kids. [↩]