Saturday, November 3, 2018

What Genealogists Know

With each previous generation you trace back, the maximum possible number of your genetic ancestors doubles. You can have 2 parents, up to 4 grandparents, up to 8 great-grandparents, up to 16 great-great-grandparents, etc.

On average, there are about four generations per century. For people born in the year 2000, their 8 great-great-grandparents were probably born around 1900. Sometime around 1800 their great-great-great-great-great-great-grandparents were born (there may be up to 128 of them). About 29 generations back, or roughly around the time of 1250-1300, the total number of your possible ancestors for that generation equals or exceeds the total population of the planet, which was about 500 million people.

What gives? Well, first of all, if all 500 million of those people were your ancestors, they would also be the ancestors of all of the rest of us, too.

Secondly, you probably don’t have every person alive back then as your ancestor. There wasn’t a lot of interracial or intercultural parenting going on back then. People were more isolated, more people lived in rural countrysides rather than dense urban areas, and people were not nearly as geographically or socially mobile as they are today. It was very common for a person to be born in and to die in the the same village or town, having lived all of her or his life there.

This means that for many, many, many, many generations, there was a lot of what most people would call today “inbreeding.” If your spouse wasn’t your first cousin, your spouse was likely a second or third cousin, or a second cousin-once removed, or even your double-cousin, etc. And as I’ve noted before, even if they weren’t marrying them, people were having children with siblings, aunts or uncles, etc. (Even if not having children together, what do you think went on, given that pubescent teens, like most children, were usually sharing a bedroom?) Not only did these things not destroy humanity, but in Europe, the Renaissance was birthed in these conditions.

Coming back to around 1800, very few people are likely to have 128 great-great-great-great-great-great-grandparents, just like very few of those people in 1800 had 128 of them in 1600. Because chances are, some of your recent ancestors were cousins, if not closer. If you marry your first cousin, you have no more than six genetic grandparents between you, instead of eight. If your parents are first cousins, you have six great-grandparents instead of eight.

If “inbreeding” was as detrimental as common misconception says, none of us would be here.

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  1. It's been shown that people tend to choose friends and spouses who are more closely related to them in terms of genotype. In one study, a pair of friends had, on average, the same genetic distance as between fourth-cousins.

    Keep in mind that we're all related to each other and that it's never a question of "whether" but simply of "how much." If my memory doesn't fail me, random Ashkenazi Jews are on average as related as ninth-cousins. Ashkenazi Jewy is actually a relatively diverse population genetically, so you can expect this genetic distance to be even smaller in other ethnic groups.

    Speaking of fourth-cousins and ethnic groups, take the Icelanders. They're all very closely related, and have a relatively high coefficient of inbreeding (compared with other Germanic Europeans). In one Icelandic study, the highest fertility rates were between third- and fourth-cousins. Incidentally, Icelandic fertility rates are well above the EU average. It's also worthy of note that the children of third- and fourth-cousins absolutely do not have higher rates of birth defects than the 2-3% found in the general population (i.e., no inbreeding depression). While correlation does not indicate causation, as a compounding variable could easily skew the data, it's conceivable that in humans there may be a fitness balance between inbreeding and outbreeding.

    1. As always, thanks for your contributions.

    2. No problems. For anyone interested in looking further into this, the proper term is "genealogical implex" or "pedigree collapse."

  2. Considering that most people rarely traveled more than a few dozen miles from home (assuming a typical village spacing of around 8 miles), you're likely related to your kinfolk far more than twice, even accounting for losses due to plagues and wars.

    Also, nth-cousin marriages in the USA only began to fade with the advent of the automobile, so even now we're only a couple of generations from those tangled family trees that most people today pretend never existed.


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