Anyone intelligent enough to post on /. is intelligent enough to know that half the DNA isn't enough.
Enough for what? To eventually grow a human, sure. But to be a person? I don't know.
Thus, if self-awareness is the measure of humanity/personhood, it's just as ok to "put down" an eighteen month old human as it is to kill an unwanted dog.
You're correct - i.e. logically speaking, either both are okay, or neither is okay.
Or, possibly, the definition of "person" is more extensive than self-awareness. But I still don't see why it should have anything to do with DNA makeup.
I fail to see the difference between the two.
It's because the definition of "person" is not strict, and for most people who haven't given it consideration, it's basically "I know it when I see it". However, surely you can imagine a hypothetical non-human person, even under whatever subjective definition you subscribe to? E.g. suppose we do determine that dolphins are "intelligent enough", after all, and devise means to communicate with them with a full-fledged language - would that not make them persons?
"Human", on the other hand, is defined entirely in strict biological terms. It's still not a strict definition if you consider corner cases (which extinct hominids were human and which weren't, for example? and at which point the result of our future evolution can no longer be called "human" and becomes a different species?), but for practical purposes, you can just do a DNA test.
Where did I indicate such a thing???
You indicated that natural rights belong to humans, and humans are defined by DNA. I don't see why such differentiation by DNA is fundamentally different from differentiating within homo sapiens sapiens by DNA; the only difference is degree. Just as you can determine the difference between humans and chimps by comparing their genes, so you can determine the difference between different human populations by looking at some genetic markers or others (and yes, there are some that correlate pretty well with black skin, for example).
And don't pretend like the fact that one case straddles species boundary and the other one doesn't makes a huge difference - "species" themselves are a rather arbitrary human construct stemming from our desire to neatly label and categorize everything, but nature doesn't really care about such things. If you want to talk about objective facts, you'll have to show a difference in quality rather than quantity of differences (or demonstrate that some quantity is a threshold meaningful for some reason other than "because I said so").
It is relevant, because with it you boil the argument down to objective facts instead of philosophical and socio-political arguments.
You can't boil the argument down without agreeing on what the argument is about. This particular one is whether personhood or humanity is the defining factor for possessing natural rights, including right to life. Yes, if you arbitrarily resolve this question in favor of humanity, then you can boil it down to objective facts - DNA etc. But that first decision is arbitrary, and not everyone agrees to it.