The sea slug, Aplysia californica, has been studied for its simple nervous system and its capacity for learning. Researchers can train Aplysia to associate a mild electric shock with a touch to its siphon, causing the animal to withdraw its gill when it is touched. This associative learning is a basic form of memory and requires the activity of a specific group of neurons in Aplysia's brain, known as the sensorimotor circuit.

The sensorimotor circuit consists of sensory neurons, interneurons, and motor neurons. The sensory neurons receive the signal from the touch, and the interneurons process the signal and relay it to the motor neurons. The motor neurons then send a signal to the gill, causing it to withdraw.

In order for associative learning to occur, the activity of the sensory neurons must be linked to the activity of the motor neurons. This is achieved by a process called long-term potentiation (LTP), which increases the strength of the synapses between the neurons.

LTP is a complex process that involves a number of genes, each of which plays a specific role in regulating the expression of LTP. For example, one gene, CREB1, encodes a transcription factor that promotes the expression of genes that are involved in LTP. Another gene, PKA, encodes a kinase that activates CREB1.

Thus, the answer to the question "How many genes does it take to learn?" is not a single number, but rather a complex network of genes that interact to regulate the process of associative learning.