system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How instincts biologically work?

So, I've always wondered how birds make their nests or how bees make their hives. It is what's called "Instinct". But in fact, I've never understood how it really works deep inside the genes and the brain of each animal. Were they "born with it" or is it transmitted culturally through the generations?
gpt
We don't really know. We know some about genetics and we know some about behaviour, but getting from genetics to behaviour is still largely unknown, although we are working on it all the time. We know that genes can control the number of receptors for certain neurotransmitters (the chemicals that send messages in your brain) so certain responses can be easier/harder depending on the number of receptors for the message. We also know that some reflexes are stored in the spine, so the signal to perform a behaviour doesn't even need to get to the brain to be processed. We also know that we can affect behaviour by deleting certain genes.  So we have pieces of the puzzle but the exact process from genes to complex behaviours like nest building is not really well understood.