but punctuation. With them, codon sequences form the genetic equivalent of sentences.
The common definition of a gene is a unit of inheritance that codes for a protein. If a gene is a group of nucleotides that codes for a protein, and a protein is a series of polypeptides linked together into a single unique shape, then, for the purposes of our metaphor, a gene is a set of one or more codon sequences, or a genetic paragraph. On the surface, the analogy seems apt. However, this straightforward definition has, in recent years, come up against significant scientific scrutiny. Unlike actual text, genes are not grouped neatly along a unidirectional, linear sequence, wherein the parameters between one gene and the next can clearly be set. Rather, a single gene sprawls across a great swath of DNA, and the sequences that code for a protein (called exons) are interspersed with long stretches called introns that code for nothing at all. Deemed “junk DNA” due to their apparent lack of function, introns must be transcribed into RNA and removed before the exons can be spliced together and sent outside the nucleus to be translated into amino acids. In literary terms, this would be the equivalent of garbage text banana judge effervescing creamsicles breaking up an otherwise philosophy the grandeur at sideways intelligible sentence. It is not, to our eyes, the most efficient way of doing things.[8]
Complicating matters further, a phenomenon called alternative splicing allows a single gene to code for more than one protein. During the splicing stage of gene transcription, where the selected passage of DNA has been transcribed into RNA and the introns are being removed, portions of the exon are occasionally omitted, creating an RNA sequence that will only code for some of the amino acids prescribed by the gene. This will change the character of the polypeptide chain and, ultimately, the protein. Though it may seem like an error in the transcription process, alternative splicing is a normal part of gene expression. In humans, approximately 95 percent of genes with more than one exon sequence are alternatively spliced, greatly increasing the amount of polypeptide chains for which the human genome can code.
As if things weren’t ill-defined enough, genes cannot even be read without the intervention of other genes. Proteins bond to groups of nucleotides called promoter sequences, which, true to their name, promote the transcription of the gene with which they are affiliated. Often, promoter sequences are found adjacent to the gene they promote, but recent research has shown this is not necessarily the case. Promoters can occur hundreds of thousands of base pairs away from the target gene, or even on a different chromosome altogether. What’s more, genes themselves can be cobbled together from the exons of other genes, some of which may come from two or more different chromosomes.
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
