So, with all this in mind, what exactly is a gene? The term “unit of heredity,” though falling out of favour in some circles, is still fairly accurate. But unlike other units of measure — inches, litres, grams, and so forth — genes are not tied to any sort of physical constant. They can be dozens or hundreds or thousands of base pairs long. They can exist across great stretches of DNA, or even across chromosomes. They can reconfigure to different lengths and code for different end products through alternate splicing. They are defined, in short, not by any precise physical characteristic beyond their approximate molecular makeup (all contain sugars, phosphates, and nucleotides), but by what they do: code for an RNA chain.[9]
The genes we will discuss over the course of this book have each been labelled “the gene” for a plethora of adverse conditions, from alcoholism to heart disease to adultery. These hyperbolic claims are usually the result of errors in translation between scientists and the public, not a deliberate misrepresentation of the facts. “Gene for cheating found” simply makes for a better headline than “scientists discover three-way causal relationship between gene, environmental influences, and an increased predisposition toward adultery.” In truth, the genes accused of causing these conditions aren’t “causing” anything. They are, at most, permitting them to happen. How? That’s not an easy question to answer, in part because we don’t yet know precisely how these genes influence our behavioural development. However, on a purely chemical level, we have a good understanding of their purpose.
The Science of Feeling Good
The name Dopamine Receptor D4 (or DRD4) refers to both a gene and the protein-based product for which it codes,[10] called a receptor. A receptor is a protein product that facilitates communication between cells. A multitude of receptors exist within the human body, each attuned to one specific molecule. In this case, the dopamine receptor binds with — as one might suspect — dopamine. Along with serotonin (which we will get to shortly), dopamine is a neurotransmitter responsible for the sense of pleasure humans derive from sex, drugs, music, sunsets, ice cream sundaes, roller coasters, warm baths, books, and any other wonderful thing you care to name.
Before we continue, perhaps we should all take a minute to thank dopamine, because it’s almost impossible to name an activity humans engage in that this chemical does not facilitate. We are, after all, continually driven by our desire for some pleasurable reward, be it in the short term (eating at a nice restaurant, watching a funny movie) or long term (exercising to feel healthier, working long hours for financial gain and/or personal satisfaction). Thanks, dopamine!
Along with its duties in the reward centre of the brain, dopamine plays an important role in cognition, voluntary movement, sleep, attention, and memory. It is a truly versatile molecule, but its ability to induce pleasure is the attribute for which, arguably, it is most famous.
The human body contains more than one kind of dopamine receptor. There are five types known at present, labelled D1 through D5. Current research has indicated the possible presence of dopamine receptors D6 and D7 as well, although the results remain inconclusive. Our focus is on receptor D4. In order to understand why, we must elucidate on an important element of heredity called an allele.
Alleles and Polymorphisms
Perhaps, while sitting in biology class or watching the news or scanning an article in a popular science magazine, you have come across these two seemingly contradictory facts: a) chimpanzees and humans share approximately 98 percent of their genes,[11] and b) children share 50 percent of their genes with their mother and the other half with their father. Both of these facts are true. Ostensibly this suggests that, genetically speaking, you are much more closely related to the chimp you saw gallivanting about the zoo as a child than you are to either one of your parents. I hope you approach this conjecture with some skepticism, as it is, to say the least, suspicious.
How can facts A and B both be valid? The problem lies with the word gene, which is being used in an entirely different manner in each case.
For fact A, “98 percent of genes” actually refers to 98 percent of the genome, meaning that, should one draw a DNA sample from a human and a chimpanzee, document every base pair of nucleotides in their possession, and match them up, those pairs would be identical 49 times out of 50. This may seem surprising, but it’s actually quite logical. For a molecule that divides and replicates so rapaciously, DNA is remarkably stable. Mutations that slip by uncorrected are rare, and when they do happen, it is often in old age, when healthy, uncorrupted versions of the mutated sequence have long since been passed down to the next generation.
Fact B takes the similarities trumpeted by fact A for granted. Using the same logic as fact A, humans are all well over 99.9 percent identical, genetically speaking. That similarity is essential to our continued survival, as it allows humans to breed with humans and not with other animals. Your genome matches 99.9 percent (or 999 base pairs out of 1,000) to your mother and your father.
So what does fact B’s 50 percent refer to? Small but critical distinctions between humans called alleles.
Shuffling the Deck
As we have mentioned, a developmentally typical human has 46 chromosomes,[12] of which he inherits 23 from his mother and 23 from his father. Each person receives two copies of chromosomes 1 to 22 (called autosomal chromosomes), plus two sex chromosomes that bear no numeric title. These latter chromosomes are called X and Y, and they determine a person’s gender. Women get two copies of the X chromosome, while men get one X and one Y. During meiosis (the creation of gametes, or sperm and eggs), one chromosome of each type is selected at random. The same goes for sperm. During reproduction or fertilization (acts instigated by dopamine, the great motivator!), the two payloads combine, creating a new organism with a full set of chromosomes, a quarter of which (give or take a chromosome or two) came from each of its four grandparents.
Think of your genes as a deck of playing cards. Each deck contains 46 cards divided into two different suits. One suit came from your mother — call it clubs — and the other from your father — let’s say spades — meaning you got 23 cards from each of them. When creating reproductive cells, your body sorts through your genetic deck, selecting one card from each of the 23 pairs at random. The result is a half-deck containing one copy of cards 1 to 23, some of them clubs and some of them spades.
Now say you meet a partner of the opposite sex, hit it off, and reproduce.[13] His (or her) half deck combines with yours, contributing its own set of cards 1 to 23, compiled at random from his own maternal and paternal suits (call them diamonds and hearts). The resulting child has a full deck of 46 cards, 23 from you and 23 from your partner. Technically, the 23 cards your child inherited from you are themselves of two different suits — clubs and spades from your mother and father, respectively — but they have become, for all practical purposes, one suit. Genetic inheritance is thus the piecemeal combination of traits from various ancestors. With each subsequent generation, the level of genetic relatedness between elder and younger is roughly halved. After as few as six generations, there is a decent chance that a person doesn’t have a single chromosome in common with their ancestor, despite the order of their nucleotides remaining, as with all humans, 99.9 percent identical.
Though we have two copies of each autosomal chromosome, the pairs are by no means identical. If they were, the entire process of chromosomal shuffling would be useless. Every functioning chromosome is largely similar to others of its type, in that it codes for the same genes, is essentially the same length, and for the most part contains the same nucleotide sequences. However, certain genes have mutations that vary from person to person, causing them to function in a different manner. For example, almost everyone has the gene necessary to determine eye colour, but not everyone’s eyes are the same shade. Some have genes that code for brown eyes, while others have genes that code for green, or hazel, or blue. Genes with multiple derivatives are called polymorphisms, and they are responsible for the astounding variety of traits between humans. Since human beings have two copies of each chromosome, they have two copies of each polymorphic gene. The precise type (or types) they possess is called an allele.
Sometimes humans have two copies of the same allele, in which case they are homozygous.