That's all there is to it. It's definitional. There's nothing metaphysical about being a human being
Other people might think that to qualify as a human being requires a sense of decency or intellect that animals don't have. Well, what about people in vegetative states or neonates? Would you call them animals? Of course not. They are humans.
I know that women have XX chromosomes and men have XY. When the man has more than one Y chromosome he's much more violent, actions I associate with being an animal. I think that there is a gene or at least set of genes on the X chromosome that makes us behave like civilized humans. It is what separates us from the other animals.
There is no single gene which causes someone to be of the species homo sapien. It is in fact multiple genes which produce the features that we recognize as human. Humans have about 20,000 protein coding genes and while not all of these are different from the genes in other animals, even slight variations in the genome or epigenetics can produce large physical differences differences enough to make humans a completely different species from chimpanzees who are still 94% similar genetically.
The simple answer is there is no one gene to define humans, or ANY other species for that matter, its a bunch of genes that were favored more in natural selection that caused humans to come about. The fact is is that as a result of multiple genes being associated with humans you can see a gradual change from our ancestors to us in the form of fossils.
There is not a specific gene that makes us human. I believe what makes us human is our ability to comprehend what is right and wrong; or our ability to communicate with one another. Our actions, attitudes, and beliefs are what make us human. These are all characteristics rather than genes that we as humans poses.
No, there is not a gene that makes us human. The thing that makes us human is our soul – that and our conscience. Without either of these we would be no different than animals in the jungle, tearing each other apart for little or no reason. Yes, there are people who are violent and similar to wild animals. These people seem to have been born without a soul or conscience. But we must realize there are mutations in every species – animals and humans.