Each according to their own gifts. They work together. There has to be a point where different kinds of cells join. It may be more correct to say that different kinds of tissues join. They all have the same basic structure, with some interesting exceptions.
Any cell is recognizable as belonging to a particular body.
When we can't tell, it is because our technology can't do it, not because a lot of people have the same genetic pattern. You get the idea. The body is known by its DNA pattern in the cells. Through baptism, we replicate. Technically, Christians are the only species to replicate by asexual meiosis.