Your Study Buddy Is Frustrated

Your study buddy is frustrated. "This book says that a single gene can make mice exhibit a certain behavior. That's ridiculous! How can you build any kind of neural circuit with just one gene?" What is your response?

A) "If the book says so it must be true. Ours is not to question why."
B) "Some genes are longer than others, over 1000kb -- they have enough information to encode a full neural circuit."
C) "Mice are a lot simpler than we are, and their brains are smaller. One gene is plenty for them to build a circuit."
D) "What the book really means is: all else being equal, one variant of a gene is associated with higher levels of the behavior than the other variant of the gene. All the other thousands of genes are still needed to develop the brain. But variation in one gene can change how a certain neural circuit will behave."