This is a common question. Since genes determine our anatomy, and influence our behavior, it’s easy to assume that every component of our bodies is determined by a gene. But this is wrong, as any biologist will tell you. In truth, there are very few components of our bodies or behavior that can be attributed to a single gene. For almost everything, multiple genes are involved.
This explanation often confuses people. How can multiple genes coordinate their actions to as to produce a given result? That would seem to be much more difficult to arrange than simply setting up one gene for each component. Right?
Wrong! I realized that there’s a simple explanation of why this is so. The key trick is to understand that the evolution of a component of the body was not a single-step process. Consider, for example, the evolution of the mammalian breast. It is not the case that one day a baby reptile hatched with fully functional breasts. Instead, the process was spread out over millions of tiny steps. First we had an animal that secreted some tiny amount of fluid for whatever reason. Then its young started lapping up the secretion, making them stronger. Over millions of years, the cells that secreted the fluid refined the formula, producing ever more nutritious milk. Simultaneously, evolutionary pressures caused more and more of these lactating cells to develop in the body. Meanwhile, the reptiles following this evolutionary path were laying less and less self-sustaining eggs that hatched earlier and earlier, with the hatchlings relying more and more on the milky secretions from the pre-breast tissue for their early growth. This evolutionary process continued until the the reptiles no longer bothered laying their eggs; they simply hatched them inside their bodies, allowed them to develop partway inside their bodies, then gave birth to the babies and continued sustaining them with milk from breast tissue.
Obviously, this was a long and complicated process spread out over many millions of years, and requiring millions of intermediate steps. Each of these steps arose from a random genetic mutation that just happened, against all odds, to further the evolutionary process.
Now, here’s the clinching question: if this process was driven by random mutations, why would all those random mutations just happen to hit the same gene over and over and over? Doesn’t it seem far more likely that those random mutations would hit different genes?
Remember, evolution is not a planned process. It’s a series of kluges piled on top of each other in a huge pile. Our entire genome is one huge collection of kluges. At no point was there the opportunity to start afresh, clean up the mess, and start over with a nice, clean structure. Hence, the long process by which mammalian breasts developed was spread out over the entire genome.
This in turn explains some apparent oddities of evolution. For example, why do men have nipples? They don’t produce milk, and they don’t have any of the cells that make milk, so why do they have nipples? The answer is clear once you realize how spread out the genetic instructions for the body are. Most of those instructions are NOT on the X or Y chromosomes — they’re all over the entire genome. There are enough of them to cause nonfunctional nipples to develop on the male chest, but not enough to build the entire breast.
Lots of other gender-specialized traits are echoed in the opposite sex by this mechanism.