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[pseudomonas]

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Comments

[ixwin]

I believe the chimp one is ambiguous, because I undestand that there are various different ways of measuring genetic similarity that would lead to different results.

[pseudomonas]

Would any of the results be <50%?

[ixwin]

Hmm. Well presumably if you were actually comparing the...expressions?...of the genes (whatever it is that two siblings would be expected to have on average 50% in common), then a given human and a given chimp would have a lot less than 50%, but I concede that's stretching the point a little. I'd tend to regard a poorly defined terms in it as intrinsically ambiguous, even if they don't affect the final answer.

Looking at your earlier comment - I know that non-coding DNA is a large chunk of the total, but I don't know how much of it is in common between species - I guess it wouldn't have had all that long to mutate since humans and chimps split, though.

[pseudomonas]

The thing that siblings have 50% in common is rare features, I think. If you have a variation in your DNA that's found at a low (negligible) level in the general population, your sib will still have a 50% probability of sharing it.

If you're looking at a feature that's present in the entire population (as is the case for almost all coding genes and most of the rest of the genome), then your sib (and everyone else) will have a 100% chance of sharing it.

[6_bleen_7]

Here there are two definitions of "same." When we say that two siblings share, on average, 50% of their (autosomal) DNA, we mean that they share 50% of their DNA identical by descent, meaning that they inherited the exact same copy of the DNA from the same ancestor. As you mentioned, identity by descent is useful for genetic studies on particular variants, although they may be common as well as rare. If you just look at the DNA sequence, without regard for the DNA's ancestry, then any two people chosen at random share some 99.9% of their genome in common (sex chromosomes excepted). This type of similarity is technically called identity in state. Sequence similarity is complicated by the recent discovery, thanks to the HapMap project, that small insertion/deletion polymorphisms are ubiquitous.