Again no one thing makes us human but a related family of things makes us so. There are indeed characteristics that distinguish the human species and some appear at a superficial level and don't require you to dissect the other to say that they're human (see http://www.statemuseum.arizona.edu/crse ... bone.shtml
). An individual might lack any of these things also and still be human. It would be a very unusual, I think, for someone to lack all of these fully-developed characteristics but if the individual has human DNA, they qualify.
In the end, you suggest that DNA is the final arbiter in the question of what is human. But you use the word "human" so freely that you know what is human before you need to understand the DNA. The sense that we need something else, like modern scientific experiments, to decide what something is, is strange because we use the words with a full idea of what that thing is.
If I ask, "what is an eye?", we would agree that it has something to do both with its form and its function. But when it comes to whole living things, the function part becomes more vague. But do we agree that there is more to being human than just plain form? If so, DNA cannot be the final arbiter. There is something that makes us human in our actions, even if we have no purpose or function. I would suggest that it is somehow tied up in doing things like learning Latin, a language for which we have no "need" in living day to day, eating, sleeping, making love, etc. Learning Latin enhances that living, I think, and it is the way in which we enhance that seems to make us different from cats, dogs, sheep, insects, plants, etc.
My daughter is mentally handicapped as well, Sceptra, and I would never suggest that she is not human. I would also not suggest that a person who is a slave is somehow less human than her master. Even a prisoner, without any tools of modern technology, acts human by the thoughts he has and the drawings he makes on the walls and floor of his cell.
I don't think questions like, "what is a human," have simple answers but only organic ones that expand and contract as part of the conversation. A dictionary is a poor tool for truly defining anything but technical terms. If we look up virtue or justice, it fails almost completely. I like to ask these types of questions because I think they are the most important thing for us, as humans, because they help us to think about how we should live. I don't adopt the position that there is no best way to live. I don't think I or anyone else will actually live in the best way, but that does not mean there isn't some ideal that we can explore and try to understand.
Michel de Montaigne wrote what he called Essays. He meant by that what the French word means, attempts. He was attempting to understand some truth via writing and he wrote as if he were having a conversation with that truth, probing it from many angles but never expecting to fully comprehend it. Today we write essays that take a "position." We prefer to pose what we see as the truth rather than make continued attempts at understanding within the same piece of writing. We make thesis statements. Others can argue with us but we then just defend our position. In my opinion, there is less benefit in this way of seeing the truth. We like to think that being open minded is a good way to be, but in that we tend to mean that there is no truth, just different perspectives. I think this is a tragedy.
And no, adrianus, I am not an "anything-ist." This type of thinking divides us more than anything. You say that my question is loaded and asked by someone seeking justification for discriminating between people. But doesn't calling someone an "ist" do the same thing? You are not to blame for this; this type of thinking is imbibed by all of us at the well of modern thought. It has become common to say that some are arrogant in assuming themselves to be superior or privileged, at the same time thinking ones own position of acceptance and equality of all is itself superior.