If the values of phenotypic traits such as running speed at a particular age, the strength of the immune response you mount to fight an infection, or anxiety level are plotted against how frequently they occur within the population, a bell-shaped curve often describes the data well. A curve of this shape is sometimes referred to as a normal or Gaussian distribution. Carl Friedrich Gauss was the mathematician who first described this curve, and it is often named after him. The reason for the bell shape is that there are few individuals with either very small or very large values of a trait of interest, with most individuals falling in the middle of the distribution. If we take running speed as our x axis, with faster runners appearing to the left of the Gaussian distribution and slower ones to the right, Usain Bolt will be far to the left-hand tail of the bell shape because he was an exceptionally fast runner as a young man. I would be further to the right, beyond the highest point of the bell. The highest point of the curve is the average, or mean, value, while the width of the bell measured at a particular point on the graph is called the variance, and it quantifies the degree to which individuals in the population differ from one another. Scientists then identify factors that might explain some of these differences between individuals, and for phenotypic trait values these factors can be split into three classes: differences in our genes (nature), differences in the environment we experience (nurture), and something called developmental noise that is currently unpredictable and is thought to be due to chance.
The statistical approach of explaining variation in phenotypic trait values allows scientists to make statements such as ‘smoking shortens lives’. There is irrefutable evidence that this is true because on average within a population life-long smokers do not live as long as those who have never smoked. However, this does not mean that smoking will lead to an early death for everyone who consumes a packet of twenty cigarettes each day. The average smoker dies younger than the average non-smoker, but we cannot accurately predict at what age any individual will die, regardless if they smoke or not. Some smokers live to old ages, avoiding the smoking-related illnesses that can kill many other smokers, and some non-smokers die young because they develop lung cancer despite never having dragged on a cigarette, cigar or pipe. Scientists can make statements about what happens on average within a group of individuals, but they cannot say with certainty what will happen to an individual. In the same way, we can say that traumatic events early in life on average result in introspective adults, but we cannot state that a child who experiences such an event will become introspective. They are just more likely to do so. It is frequently impossible to say with complete confidence that the reason a person has a particular value of a phenotypic trait is down to nurture, nature or chance. It is consequently impossible to state definitively why each of us is the way we are, but we can identify probable causes and roles for genes and the environment.
I have already described genes and summarized how they work, but it is worth briefly recapping. A gene is made up of long strings of molecules called nucleobases of which there are four types, A, T, C and G. Reading your genetic code would rapidly get monotonous as it is a very long string of these four letters. Nucleobases are grouped into threes, with each group of three instructing the molecular machinery in your cells to grab a particular type of amino acid – a type of molecule made from carbon, oxygen and nitrogen atoms arranged in a particular configuration that enables them to form chains.
Imagine the first three nucleobases in a gene are CCC. The triplet tells the molecular machinery to grab an amino acid called proline from the molecules in your cells. The next triplet in the gene reads GCA, which codes for the amino acid alanine. The molecular machinery selects an alanine molecule and joins it to the proline molecule, with this process continuing until a stop sequence such as TGA, TAA or TAG is encountered which tells the molecular machinery that the protein’s production is complete. The molecular machinery that translates the genetic code to a protein contains lots of steps involving many types of molecules that I do not describe here.
The chain of amino acids then folds itself to create a protein. Proteins run our metabolism and are used to build our bodies. Something as simple as a change in a single amino acid in the chain can alter how effectively a protein does its job. For example, if the nucleotide sequence of a gene is changed even just a little, the sequence of amino acids in the chain may change, and the effectiveness of the protein produced may be impacted. If a nucleotide triplet CCC were to change to CAC, a proline in the chain would be replaced with a histidine and the chain might no longer fold as it should. When a mutation happens that changes the sequence of amino acids in the chain, the protein might stop working altogether, its function may be slowed, and in occasional cases it may even work more efficiently. When the order of amino acids in a chain is altered due to a change in the sequence of nucleobases, a genetic mutation has occurred. Each of us has different mutations – different variants of the human genetic code – and these differences can contribute to differences between us in phenotypic traits such as how outgoing we are, our birth weight, adult height, or our ability to run 100 metres quickly at age twenty.
I know of one genetic mutation I have because it prevents a protein functioning properly. I have a mutation in a gene called GPR143 that produces a protein with the snappy name of G-protein coupled receptor 143. The protein consists of 404 amino acids, and in my case at least one of these is not what it should be due to a mutation. I do not know how the mutation I have alters the order of amino acids in the protein the gene codes for, but the protein does not function as it should. In the database of human genes and what they do, GPR143 is described as encoding for ‘a protein that binds to heterotrimeric G proteins and is targeted to melanosomes in pigment cells. This protein is thought to be involved in intracellular signal transduction mechanisms. Mutations in this gene cause ocular albinism type 1, also referred to as Nettleship–Falls type ocular albinism, a severe visual disorder.’ In more accessible language and drawing on descriptions of how the eye develops, this means that when the protein works correctly it acts to add colour to a small group of cells in the back of the eye called the fovea. My fovea lacks colouration, leading to a genetic condition known as ocular albinism. Such a small effect – a few uncoloured cells – might sound trivial, but it does mean my eyes wobble, and this is the cause of my poor vision. My eyes do this because the colour that most people have in the fovea helps stabilize their vision, and my vision is not stabilized as it should be. My mutation means I have about 25 per cent of normal vision, and this means I can only read down to the third row of the eye charts at the opticians. My eyes have always wobbled and always will. It hasn’t had too many detrimental impacts on my life other than that my eyesight is not good enough to allow me to drive safely. I must live within cycling or walking distance of work, and I am always the designated drinker if Sonya drives us to a social event.
The mutation I have means there is an error in my genetic code. Your genetic code is a self-assembly manual on how to build you, and the human genome is the generic self-assembly manual on how to build a person. My instruction manual to assemble a human eye has an error in it. Most of the instructions are correct, but the bit that says ‘now add colour to these cells’ is missing. My copy has a typo in gene GPR143, and this means my eyes did not develop as they should. To understand what this means I turn to the science of developmental biology, the science of describing how we grow and how organs such as our heart, kidneys and eyes are formed.