Complex multicellular organisms such as you and I start out as a single cell that then divides. Such cell divisions keep occurring until a small ball of a few tens of identical cells exists. From that point onwards, different cells start to develop in different ways. Those on the outside of the ball will go on to develop into skin, while those in the centre will become our internal organs. The way that cells ‘know’ how to develop is due to chemical signals. The concentrations of these chemicals determine when to turn particular genes on or off, and this determines how each cell develops and whether it will become a nerve cell, a muscle cell, a neuron in the brain or a blood cell in the veins.
As development continues, cells start to move around within the developing embryo, migrating to particular areas. In the brain, for example, the end of a neuron is guided into its final position, where it joins with other cells by means of things called neuronal growth cones, that are a little bit like molecular tractors that move along concentrations of particular chemicals in the developing brain. These chemicals act like signposts, telling the molecular tractor to keep going, head right or left a little, or stop.
By the time we are fully grown, our body consists of over 30 trillion cells. Many of these cells migrated into their final positions within the developing body following gradients of chemicals. These chemicals are produced by genes, or by reactions involving the proteins that the genes code for. Your genome does not contain a map of where each cell in your body will be and how they link to one another. Instead, it consists of a set of instructions for when each protein should be produced in each cell type, and when the rate of production of that protein should be turned up or down or turned off. These on and off instructions result in gradients of chemicals in your developing body, and these chemicals allow your body to construct itself. Your genome codes for instructions that state ‘turn on if such-and-such a chemical is below a certain concentration, and turn off otherwise’.
Development is complex, and this means that most phenotypic traits are not determined by a single gene. Your adult height is determined by how long and how quickly your bones grew. Tall individuals have produced longer bones than those that are smaller because their bone cells continued to divide for longer during development or they divided more quickly. Growing bones is a complex task, with genes controlling the rate at which they develop, the directions in which they grow, the shapes they develop into, and the length of time they grow for. Your height is consequently not determined by one gene but by very many that control all these aspects of bone growth. There are many genetic ways to be tall or short, thin or fat, funny or serious.
When scientists discovered genes they did not have this understanding of how they control development. Early in the genetic revolution, when scientists discovered they could read DNA sequences, some biologists thought they would find single genes for nearly all our phenotypic traits. A gene for intelligence, another for life expectancy, and a third for the ability to play cricket. Some researchers dreamed of using genetic engineering to make people smarter, longer-lived, or better batters at cricket or baseball. Many benefits could be seen if genes were to work like this, with all sorts of diseases potentially becoming treatable. Perhaps a murderous psychopath could be turned into a thoroughly decent chap following some genetic engineering. But it was not to be. As we have learned more about the way our genes work and our bodies develop, we now see our genomes as self-assembly manuals, explaining how to build a human from a single, fertilized cell. There is no genetic homunculus determining your exact developmental trajectory from conception. The values of most phenotypic traits are controlled by lots of genes, and my genetic condition, ocular albinism, is unusual in that the extreme phenotypic trait value I have for my eyesight can be attributed to a single gene. Because only one gene causes ocular albinism, it could be a candidate for gene therapy. Despite this, my mutation does not operate completely independently, and although all males who have the mutation develop poor eyesight, we are not all equally partially sighted.
There are many members of my extended family who carry the mutation in the GPR143 gene and several men who have poor eyesight because of it. Although we all have worse than average eyesight, some of us can see better than others. The mutation in the GPR143 we have means we all lack pigmentation in our fovea and that impacts our sight, but other genes involved in the development of the eye mean that the impacts of the mutation on our eyesight differ between us. These other genes contain instructions on which types of cells to produce and where they should be positioned. No gene works independently when building a body or a phenotypic trait such as adult height, and this means that identifying how a particular gene influences the value of a particular trait is usually very hard. The reason that biologists know GPR143 influences vision is that all males who carry my mutation have eyesight that is a long way from the average of the bell curve, and geneticists have worked out what the protein the gene produces does. Biologists have also worked out a lot about how the eye works and have shown how important colouration in the fovea is for normal eyesight.
The self-assembly instruction manual for a human body is so complex, and involves so many steps and chemical gradients, it is perhaps not surprising that if you run the identical code multiple times it does not produce exactly the same result. Variations in chemical concentrations that we cannot predict caused by molecules bouncing around off one another within and between your cells as you develop mean that if multiple bodies are produced by the same genome, they are not completely identical but can have different phenotypic trait values. Biologists refer to these unpredictable impacts on development as developmental noise, and they have produced a number of clever methods to explore the impact of chance on determining our phenotypic traits. For organs that come in pairs like the kidney or feet, biologists can compare how these differ within individuals. Many of us have better eyesight in one eye than the other, with my left eye being slightly less bad than my right. Each eye develops in the same way, using the same genetic code and developmental steps, so any differences are due to events that we cannot predict and that consequently appear to be random. Biologists can also use differences in phenotypic traits between genetically identical twins to investigate the role of developmental noise.
Identical twins have identical genomes and they often look very similar, but there are differences between them that their parents are usually very quick to spot. These differences are due to developmental noise caused by small differences in the concentration gradients that developed within each twin as they grew. Such developmental variation means that if you were to create a hundred clones of yourself, they would not be perfectly identical. They will each have developed in slightly different ways, with each having a slightly different height, or face shape, or brain structure. Some traits seem to be more susceptible to developmental noise than others, with some psychologists claiming that some bits of personality may be strongly impacted by developmental noise. A hundred identical clones of you might vary in their degree of extroversion, with their individual scores producing a bell curve, with one or two being extrovert, one or two being shy and the rest being somewhere in between.
I have described how two processes, genetic differences between us and chance, can influence who we are, but there is a third process to consider, the environment. The term ‘environment’ captures a lot. It includes, among other things, the country where you were born, the month and year of your birth, the culture you experienced while growing up, the altitude at which you spent your youth, your socioeconomic class, the amount and types of food you ate as a child, the people you grew up surrounded by, and events you have experienced. Identifying which aspects of the environment to measure and to focus on in scientific research is hard.