The Pleasure Instinct: Why We Crave Adventure, Chocolate, Pheromones, and Music by Gene Wallenstein

and subsequent protein production are not transmitted from generation to generation genetically. None of these alterations in gene expression is incorporated into the sperm or egg, and therefore are not heritable.All changes in gene expression that result from learning or being exposed to experiential/environmental factors are transmitted culturally rather than genetically, and they clearly have a profound impact on the way brains develop.
    The fact that genes have essentially two functional components has important implications for development and the relationship between nature and nurture. Being familiar with the way genes really work makes it easier to see why most biologists have long ago given up on the nature versus nurture debate as a false dichotomy. The genes that code the way brains are built do not contribute to development unless they are transcribed and expressed. Hence, experience is an essential part of development even at the level of the gene.
    How do genes build brains? As I write this chapter my wife is four and a half months pregnant, and every day brings new questions about little Kai’s development. The typical adult human brain has about 100 billion nerve cells or neurons. Each neuron connects to thousands of others, resulting in about 10 14 (a 1 followed by 15 zeroes) different connections. How, then, do the 25,000 or so genes identified by the Human Genome Project code such a combinatorically large and complicated system? Clearly, since the numerical differences are so great, genetic information does not uniquely specify where every single neuron resides or where each of its thousands of connections will terminate. Instead, genes specify more general rules for neuron development and migration.
    At the earliest stages of development, we start off as three primitive cell layers: endoderm, which consists of cells that eventually line our internal organs and vessels; mesoderm, destined to become the major structural components of the body, including bones and muscle groups; and finally, ectoderm, which becomes the central nervous system, skin, hair, and nails. Kai’s entire mental world—his thoughts, emotions, sensations, and perceptions—emerge from this thin sheet of cells. Within the first few days of gestation, the primitive cell layers elongate and fold into a cylindrical tube called the notochord, the progenitor of Kai’s spinal column. This process recapitulates the earliest event in the evolutionary transition from invertebrate to vertebrate forms that occurred more than 600 million years ago.
    Once the notochord is formed, it guides the ectoderm layer, which progresses through a series of well-defined stages, first thickening and then folding in on itself to form the neural tube. At about nineteen days into gestation, just about the time when Melissa and I first learn she is pregnant, the earliest form of Kai’s future brain and spinal cord begin to emerge through a process called neurogenesis. During this period, the front end of Kai’s neural tube develops three enlargements, which eventually become the two cerebral hemispheres and the brain-stem. The neural tube then goes through a rapid growth spurt, where the entire cycle from cell division to cell division takes place in about an hour and a half. Some of these precursor cells are destined to become neurons, while others will mature into glial cells, which serve a variety of supportive functions in the brain.
    As cell division and replication continue, the three enlargements begin to take on more detail, eventually forming all the major components of Kai’s brain. At two months into gestation he is little more than two inches long, yet all of his major brain structures have begun to take shape, including the elementary forms of the medulla, pons, and midbrain, which combine to form the brain-stem; subcortical structures such as the thalamus, hypothalamus, and basal ganglia; then a bit more slowly, the allocortex; and even more