There is nothing peculiar about genetic determinism which makes it particularly sinister. Also, when somebody announces that they have discovered a gene for let's say aggression or religion, this does not have a deterministic force in the sense of irrevocable determinism, any more than discovering that a particular chemical in a diet has an effect. You might find that people who eat red peppers are more aggressive than those who don't. I have no evidence for that, but you could find some such thing, and that's not deterministic either. That too will be a statistical effect that will be added in with all the other effects.
Genes are to be thought of like that.
The only reason that Darwinians talk about genes so much is that in order to do Darwinism they have to be looking at those aspects of individual variation in populations which are genetically influenced. So we're not talking determinism, we're talking statistics, we're talking analysis of variance, we're talking heritability. That leads on to my next question. This is a convention. It means a gene such that if you change that gene to its allele, you might see a corresponding change in the phenotype.