Manolis Kellis

It's just extraordinary to sort of see these dramatically different... Like, I mean, my wife and I, you know, are very different from each other, but we also have, you know, 6 million variants, 6 million loci each, if you wish.

If you just look at common variants, we also have a bunch of rare variants that are inherited in a more Mendelian fashion.

And now you have...

an infinite number of possibilities for each of the kids.

So basically it's two to the six million, just from the common variants.

And then if you like layer in the rare variants.

So let me talk a little bit about common variants and rare variants.

So if you look at just common variants,

They're generally weak effect because selection selects against strong effect variants.

So if something like has a big risk for schizophrenia, it won't rise to high frequency.

So the ones that are common are by definition, by selection, only the ones that had relatively weak effect.

And if all of the variants associated with personality, with cognition and all aspects of human behavior were weak effect variants, then kids would basically be just averages of their parents.

If it was like thousands of loci, just by law of large numbers, the average of two large numbers would be very robustly close to that middle.

But what we see is that kids are dramatically different from each other.

So that basically means that in the context of that common variation, you basically have rare variants that are inherited in a more Mendelian fashion that basically then sort of govern likely many different aspects of human behavior, human biology, and human psychology.

And that's, again, if you look at sort of a person with schizophrenia, their identical twin has only 50% chance of actually being diagnosed with schizophrenia.

So that basically means there's probably developmental

exposures, environmental exposures, trauma, all kinds of other aspects that can shape that.

And if you look at siblings, for the common variants, it kind of drops off exponentially as you would expect with, you know, sharing 50% of your genome, 25% of your genome, you know, 12.5% of your genome, et cetera, with more and more distant cousins.

But the fact that siblings can differ so much in their personalities that we observe every day, it can't all be nurture.