I keep telling people there's junk in them thar genomes, and lots of it. ERV has a nice summary of some recent work in which a detailed comparison of the distribution of LINE retrotransposons (selfish DNA that does nothing but insert copies of itself into your DNA; they make up about 20% of your genome) was made — and what they found was that variations were common, new insertions were relatively frequent, and that they can be used to map out human relatedness (although we've known that one for a while).
LINEs are super useful for determining/tracing human ancestry. As Im sure you all have heard me mention before, new bits of junk that arent 'supposed' to be there make excellent connect-the-dots pictures for ancestry (this isnt new, but its still neat).
Any two people had ~285 differences in human-specific LINEs (148 minimum, 422 maximum). I have some you dont have, you have some I dont have.
They did not check to see whether these LINEs were homozygous (in both copies of your genome). There is probably even more diversity here-- I have some LINEs you dont have, but some of them are just on one of my two copies of DNA in each cell. Since we only pass on one copy of our DNA to our offspring, this has an effect on how fast/whether new LINEs are fixed in a population or not. Genetic drift, w00t!
The older the LINE, the more likely we all have it in common (we are all related!).
Brand spanking new LINEs pop up in ~1/140 births. So, of the ~6 billion people alive today, there are ~30 million new LINEs between us all. If all of these junk LINEs are precious and specially created by Teh Designer, all of us without the new sacred LINEs should be dead. We aint. So...
Cool stuff. There's a lot of churning over of these things, which is one indication that they're non-essential junk.
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