We each have 23,000 genes, most of which code for proteins. Every cell in the body has a complete copy of the genetic (DNA) sequence, comprised of 3 billion letters - nucleic acids - grouped into 23 chromosomes. In other words, genes are sequences found in chromosomes. Genes (DNA) act as templates for the sequence of amino acids that comprise proteins.
If you string together a series of amino acids into a protein, they begin to fold into a characteristic 3-dimensional shape. At that point, proteins can be used for many purposes. As building blocks to construct the body. As signals. Or as catalysts, called enzymes.
Enzymes catalyze chemical reactions in the body, whereby molecular metabolites are brought together, or split apart. The products of one reaction feed into the next reaction, in complex metabolic pathways.
Development thus proceeds, building structures in the body, sending and receiving signals (for example, hormone signals). The receiver is usually a specialized protein, often on the surface of a cell, in which case the cell has a specific response. In other words, if the cell doesn't have the built-in receiver (surface protein) it won't notice and respond to the signal (hormone).
The brain develops similarly. There are too few genes (only 23,000) to specify the precise location of each of the 100+ billion neurons in the brain. So brain development relies on construction of repeating units (there are 2 million cortical columns - processing units - each with 100,000 neurons).
I’m always most interested in the development of the brain itself, a process known as neurulation. The neural cells continue to differentiate into specialized neurons in the brain. A sort of chemical grid, or map, guides them as they migrate to their proper position, connecting with other neurons along the way.
There are two types of development in the early brain. Activity-independent mechanisms (such as differentiation, migration and axon guidance) proceed according to genetic programming, independent of the environment (neural activity and sensory experience).
But once the neurons are in place, with their axons connected to other neurons, activity-dependent mechanisms of development (influenced by the environment) can begin. Neurons do not connect directly with each other. Instead, there is a gap, called a synapse, between every neuron, where chemical signals pass back and forth. The chemical signals are vulnerable to being intercepted by drugs, like antidepressants and antipsychotics, because the brain is designed for exploitation by "global signals" and external agents (if not, the neurons would connect together directly).
The synaptic connections between neurons change over time, influenced by our experiences. But those are not just any experiences. They are the specific experiences for which our development was programmed to respond, like an elevator which is programmed to respond to "button presses" but not loud screams. Some genes are programmed to switch off (semi-permanently) in some people under certain environments, which is why identical twins (people with the same genes) may differ. One twin may have a slightly different experience in his life, for which a particular gene is programmed to switch off, so now the twins have, in effect, different (active) genes!
We learn skills, perfect them, and make new memories – all of which are possible by the activity of the brain. The genetically developed substructures in our brain help us remember new things (hippocampus), motivate us (amygdala), allow us to plan (frontal lobes), and learn language and other skills. Evolutionary psychologists believe we have evolved hundreds of specialized regions in the brain, each triggering specialized human desires, motivations and behaviors. Since each of us has different gene variants (probably leading to a different amygdala structure), no two people are motivated by the same things.
Genes develop the body and brain. But the body has some of the properties of a mechanism, like a toaster. It has an internal state (a toaster has on-off states). It's behavior depends on environmental stimulus (a toaster needs someone to press the lever). Genetic development is unlike a toaster, in that it assumes input from the environment for tuning purposes.
Saying "genes cause human behavior" must consider this developmental intermediary. Genes are blueprints for development, and the resulting body/brain are what cause the behavior. It's somewhat indirect. But that doesn't make behavior any less innate.