Just a Few Human Cells
Mice move over: Human-chimp chimeras are exactly what the Elysians make in Daughter of Elysium.
Of course, this story has all the elements of the “forbidden science” theme: It’s not done here (of course not) but in China (how many reports of humans cloned in China, back in the day). If human-chimp chimeras are actually made, it shouldn’t be hard; actually much easier than human cloning. Human-mouse chimeras have been made already, though not past the stage of early embryos.
Other animal chimeras have been grown to maturity, for example a goat with a sheep makes a sheep-goat chimera. This is different from a hybrid, that is a sheep inseminated by a goat or vice versa, where every cell in the body has half sheep chromosomes and have goat. In a chimera, each cell comes from one parent or the other, but the cells are gemished throughout the organism in various ways. Chimeras are interesting because their sex cells come from one “parent” or the other. So a human-chimp chimera could theoretically produce human sperm or egg. Of course scientists say they won’t make chimeras that can do that.
Why make these chimeras? The same reason science fiction always offers for human clones: A source of transplantable organs.
So how much human does the chimera have to be before it expects human rights? What percentage? What organs? A human spinal column? Oops.
According to the news report, if mice-human chimeras were to be born, “it is very unlikely the animals, if brought to term, would take on human-like behaviour, but the animals might not behave like “normal” rodents.”
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Ultraphyte 
What controls the size between the cell origin? If human cells are added to a mouse, do they fully mix or not? Do human cells that develop into human organs try to become human-sized, or are they constrained to mouse size? What determines any development conflict> Human brain cells are programmed to divide more than mouse brain cells. Does teh mouse brain override this somehow, or do the human cells try to divide sufficiently to be unconstrained by a mouse skull? Currently, the ratio of human cells in a chimera is very low, but what happens if they are increased? Pigs and humans are of comparable size to obviate the questions I ask, but what of when human-mice chimeras are made?
The size of cells, mouse and human, is about the same. The difference in size of the animal has to do more with overall number of cells.
When a few human cells are mixed with a mouse embryo, the overall embryo development is that of a mouse. The human cells mix in thoroughly, as far as one can tell; this is recent research, so I don’t think we can know whether or not every normal molecular pathway is formed .
The relative rates of cell proliferation depend on the details of the experiment. I don’t think the molecular basis is yet known.
In the case of other animal combinations, I don’t recall details, but I think it was hard to predict which cells would outgrow others. The sheep-goat chimeras had plenty of both types.