Interjecting opinion as someone from the peanut gallery who agrees with what Tristram Evans wrote before, I would say that living things will have feelings, but if you cut & paste parts with bioengineering, it may have side effects on how they feel. Non-engineered living things have nervous systems that run throughout their bodies and grew there from itself without experiencing bioengineering, and the whole nervous system (not just the brain) stores emotions and memories. People who have organ transplants often have various sorts of somatic and personality adjustments. So I'd expect bioengineering to affect how they think and feel.
I agree that the engineering would affect *how* they think and feel. However, I am suggesting that they still *could* think and feel. Ultimately, the question is, what are feelings?
1) Are they produced by neurons at all, or is there some immaterial spirit that creates them?
2) Are they unique to only naturally-grown neurons? Or could artificially-generated neurons in the same configuration still produce feelings?
3) If artificial neurons could work, then do they have to have exactly the same water/lipid/protein mechanical structures that we do - or could there be differences and yet still produce feelings?
Within nature, we can see a clear spectrum from unfeeling life (such as bacteria and lichens) to life with only rudimentary nerves like jellyfish or flatworms, working up to complex nervous systems like chickens and humans.
I am inclined to think that our sentience is a result of the higher-order structure of our nervous system - i.e. how our senses, memory, and thoughts interact. I don't think it is something uniquely tied to the water/lipid/protein structure of neurons.
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To connect this back to games, there are a bunch of potential characters in science fiction games, including:
1) Human clones and/or humans with genetically-engineered DNA.
2) Biological constructs ranging from Frankenstein's monster to the biological replicants of Blade Runner.
3) Mixed biological and mechanical constructs, like part-flesh Terminators or DARYL.
4) Purely mechanical constructs like Star Wars droids.
5) Nanotech constructs, like the T1000 Terminator.
Any or all of these might be considered slaves. It could be interesting to ask how attitudes differ among these.