It is not about whether or not genes can encode behavior, they obviously can. But explaining behavior with genetics has never actually been done, and most attempts of doing so have been pseudo-scientific race science with the goal of showing the superiority of the white race.

Yes we can select a behavioral trade and breed animals that are more likely to behave in a certain way in a given situation. Those animals are usually also trained from birth to behave in that matter, demonstrating how important environmental interaction is to explaining behavior. Selective breeding is a proof that some behavior is heritable. But heritability does not mean that you can explain behavior with genetics, it just means that some behavior is more likely within some population, regardless of whether or not genes are the reason for the variety. Twin studies, even if they were valid and unbiased, fail to account for that, and are therefor not evidence for the wild claims of behavior genetics.

But it gets worse. Like I said before, twin studies are inherently biased and they don‘t actually show an accurate estimate of heritability as behavior geneticist claim. Twins are not a random sample of the population, twins share the same environment at least until the first minutes after birth (and quite often months or even years after birth), they often interact with each other, and even if they are separated soon after birth, they are more often adopted into similar (often high income) families. In short twin studies suffer from bad statistics resulting from junk-in junk-out.

And it gets even worse, because even looking past the fact that heritability does not offer any evidence for behavior genetics, and the fact that twin studies are fraught with bad statistics, the fundamental assumptions of twin studies are wrong. The human genome is not static throughout the live of the individual, our own genes only account for less then half of our genetic mass (the rest are from microorganisms some are there from birth, others leave and enter our bodies frequently, some we might even trade genes with, and a lot of them affect our behavior).

If you are interested, here is a 2014 report debunking twin studies as proof for behavior genetics: https://onlinelibrary.wiley.com/doi/10.1111/1745-9125.12036

EDIT: As for the claim that “[w]e already know that genetics predispose people to certain conditions like schizophrenia.”, that is not so clear either: https://slate.com/technology/2016/02/schizophrenia-genes-fou... and if you want an unopinionated review of the current status of the genetic theory of schizophrenia: this is a nice one: https://pmc.ncbi.nlm.nih.gov/articles/PMC7465115/ and concludes:

> From the impressing 80% heritability stated by Sullivan, very little has been pinpointed and confirmed in live models or come even close to the ultimate goal of targeted therapy. [...] Many confounding variables still plague all levels of testing starting from sample sizes, absence of negative studies of insufficient follow-up for high credibility.