There are many contexts in healthcare where it's considered socially acceptable not to be strictly honest and accurate.

Psych drugs commonly say, or used to say, in the info leaflets something like they "correct imbalances of certain chemicals in the brain".

This is completely made up, and apart from the vagueness, obviously not reasonable when you consider that such drugs usually take a considerable amount of time to have their full effect.

But it's a customary "white lie" (or rather a fiction) to try to sooth patients and keep them confidently on meds. It's not a big secret, or a scandal, but an attempt to manipulate hopefully trusting people for their own good.

As another example of the dynamic, doctors treat patients who are inevitably going to die in the near future, and if they sense the patient is still holding out hope and still wants treatment, they will probably avoid accurately communicating the reality, the number of weeks or days left, and instead say things about progress and remission until the end.

I don't know what context "the gene" for bipolar disorder came up in, but could it possibly be that your family member was obsessed with "the gene" and the doctor humored them? You've heard of sugar pills?