Heredity

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People spend a lot of time worrying where they come from and how they got here. There are lots of groups or web-sites where you can go to trace your genealogy. Past life analysis — one of my favorite woo-woo things— would suggest we all came from Cleopatra or some other notable figure of history. In my own family, my father and brother worked hard to examine our family tree, tracing us back eight generations to an immigrant from Yorkshire.

Of course, that distant ancestor contributed less than 1% of my own personal DNA. The math is easy. My parents each gave half; my grandparents ¼. My great grandparents just one eighth and so that means my original Trenholm antecedent contributed about 1/128th. Frankly he could have been anyone.

Except he couldn’t. That past life thing may have some merit. In reality, most people from a couple of hundred years ago have exactly zero descendants. Think of it this way. Three neighbours each have 5 children. Since this is 200 years ago when infant mortality was very high, one of the neighbours loses all of her children before they reach adulthood. She has no descendents today. The other two are luckier. One has two children reach adulthood and the other four. The second one now has twice the potential descendents of the first. And of course further deaths before puberty in subsequent generations could result in only one of the neighbours having living relatives today. Or neither of them could.

Genetic studies suggest that fewer than 2000 people alive on the planet 2000 years ago (when there were hundreds of millions of people) have living descendants. Things either work out or they don’t. It is this odd law of nature and the peculiarity of arranged marriages in the European royalty that results in Queen Elizabeth II being a linear descendant of Mohammed the Prophet(though the Portuguese royal family). Boggles the mind (but makes one think how trivial racism is).

So when I read that studies suggest that epigenetic changes — that is changes to genes and gene expression that occur after conception — could be passed on from generation to generation, I was skeptical (and remain so). In the first place it is all based on gene studies of mice. And while mice are important research animals they aren’t people. Most successful drug studies in mice never produce a real result in humans. We’re just too different. As well, the mechanism is unclear, involving the binding of certain chemicals to DNA. These chemicals are generated by stress and may be a factor in how genes express as we age. Nothing revolutionary here. We’ve known for a long time that the environment impacts gene expression. For example, birth placement of identical twins (i.e. location in the womb) can determine whether the gene that causes diabetes expresses itself in the child. So one twin can have the disease while the other is healthy.

But to suggest that the negative experiences of your great grandmother appear in your behavior today not only has to overcome the dilution factor (she is only 1/8th (or 12.5%) of who you are) but also needs a mechanism related not to adult cells but to gamete cells (sperm and eggs) which are much harder to change than most.

And that’s ten minutes.

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