Summary

A great entry point into the human genome, how it works and what genes do (and don’t do). Still useful 25 years after publication.

Key Takeaways

• There are none so blind as do not wish to see.
• Evolution has no pinnacle and there is no such thing as evolutionary progress. Natural selection is simply the process by which life-forms change to suit the myriad opportunities afforded by the physical environment and by other life-forms.
• The main purpose of genes is to store the recipe for making proteins.
• Anticipation means that the longer the repetition, the longer it is likely to grow when copied for the next generation.
• Pleiotropy: a technical term for multiple effects of multiple genes.
• Our immune systems are set up in such a way that they ‘expect’ to be educated by soil mycobacteria early in childhood; when they are not, the result is an unbalanced system prone to allergy.
• More damage has been done by false negatives than by false positives.
• Simplicity piled upon simplicity creates complexity. 
• Heritability does not mean immutability.
• As you grow up, you gradually express your own innate intelligence and leave behind the influences stamped on you by others. You select the environments that suit your innate tendencies.
• Sexual selection is thus an expression of sexual antagonism between genes for seduction and genes for resistance.
• The main purpose of most genes in the human genome is regulating the expression of other genes in the genome.
• Cortisol levels rise in response not to the amount of work you do, but to the degree to which you are ordered about by other people.
• Testosterone is just as good at suppressing the immune system as cortisol.
• You can alter your serotonin levels by altering your eating habits.
• Your brain chemistry is determined by the social signals to which you are exposed.
• The genes can be induced to change by voluntary, free-willed, conscious action.
• Heart disease is a preventable and treatable condition. How many lives could be saved, and early heart attacks averted, by simple genetic diagnosis to identify those at risk and target treatment at them?
• Down-syndrome babies are generally born to older mothers. 
• Children probably have more non-genetic effect on parents than vice versa.
• People get their personalities from their genes and from their peers, not from their parents.
• Human behaviour is unpredictable in the short term, but broadly predictable in the long term.

What I got out of it

Genome is, in my opinion, Matt Ridley’s best book. Written in 1999, it’s likely outdated in 2022, but I still learnt a lot about biology, genetics, evolution and heredity and changed much of my thinking.

Some things I learnt:

• Our immune systems are set up in such a way that they ‘expect’ to be educated by soil mycobacteria early in childhood; when they are not, the result is an unbalanced system prone to allergy.
  • This made me question how often I should wash my hands.
• Genes need to be switched on in order to work.
  • This blew my mind. I had never thought about this. In Nature Via Nurture, Ridley covers this in greater detail.
• Heritability does not mean immutability. The genes can be induced to change by voluntary, free-willed, conscious action as well as by external events (via hormones for example).
• The main purpose of most genes in the human genome is regulating the expression of other genes in the genome.
• People get their personalities from their genes and from their peers, not from their parents.
• Genes are not there to cause diseases. Gene mutations can lead to disease, and sometimes there is a balancing effect between resistance to one disease at the expense of being susceptible to another disease.
  • Another point that blew my mind. I’ve never looked into genetic diseases or how they operate…this definitely made me think about how something is phrased and the (wrong) impression that phrasing can give.

Summary Notes

Preface

Imagine that the genome is a book: 

• There are twenty-three chapters, called CHROMOSOMES. 
• Each chapter contains several thousand stories, called GENES. 
• Each story is made up of paragraphs, called EXONS, which are interrupted by advertisements called INTRONS. 
• Each paragraph is made up of words, called CODONS. 
• Each word is written in letters called BASES. 
• There are one billion words in the book, which makes it longer than 5,000 volumes the size of this one, or as long as 800 Bibles. 

Chromosome 1 – Life 

Living things produce approximate copies of themselves: rabbits produce rabbits, dandelions make dandelions. But rabbits do more than that. They eat grass, transform it into rabbit flesh and somehow build bodies of order and complexity from the random chaos of the world. They do not defy the second law of thermodynamics, which says that in a closed system everything tends from order towards disorder, because rabbits are not closed systems. Rabbits build packets of order and complexity called bodies but at the cost of expending large amounts of energy. In Erwin Schrodinger’s phrase, living creatures ‘drink orderliness’ from the environment.

Chromosome 2 – Species 

There are none so blind as do not wish to see.

The human species is by no means the pinnacle of evolution. Evolution has no pinnacle and there is no such thing as evolutionary progress. Natural selection is simply the process by which life-forms change to suit the myriad opportunities afforded by the physical environment and by other life-forms.

Imagine you take a photograph of a chimpanzee:

• To develop it you must put it in a bath of developer for the requisite time, but no matter how hard you try, you cannot develop a picture of a human being on the negative by changing the formula of the developer. 
• The genes are the negative; the womb is the developer. 
• Just as a photograph needs to be immersed in a bath of developer before the picture will appear, so the recipe for a chimpanzee, written in digital form in the genes of its egg, needs the correct milieu to become an adult – the nutrients, the fluids, the food and the care – but it already has the information to make a chimpanzee. 

The same is not quite true of behaviour:

• The typical chimpanzee’s hardware can be put together in the womb of a foreign species, but its software would be a little awry. 
• A baby chimpanzee would be as socially confused if reared by human beings as Tarzan would be if reared by chimps. 
• Tarzan, for instance, would not learn to speak, and a human-reared chimp would not learn precisely how to appease dominant animals and intimidate subordinates, to make tree nests or to fish for termites. 
• In the case of behaviour, genes are not sufficient, at least in apes.

Chromosome 3 – History 

The main purpose of genes is to store the recipe for making proteins. 

It is proteins that do almost every chemical, structural and regulatory thing that is done in the body: 

• They generate energy
• Fight infection
• Digest food
• Form hair
• Carry oxygen 

Every single protein in the body is made from a gene by a translation of the genetic code. The same is not quite true in reverse: there are genes, which are never translated into protein, such as the ribosomal-RNA gene of chromosome 1, but even that is involved in making other proteins. Garrod’s conjecture is basically correct: what we inherit from our parents is a gigantic list of recipes for making proteins and for making protein-making machines – and little more.

Chromosome 4 – Fate 

Anticipation: it has been known for some time that those with a severe form of Huntington’s disease or fragile X are likely to have children in whom the disease is worse or begins earlier than it did in themselves. Anticipation means that the longer the repetition, the longer it is likely to grow when copied for the next generation. We know that these repeats form little loopings of DNA called hairpins. When the hairpins unfold, the copying mechanism can slip and more copies of the word insert themselves.

Chromosome 5 – Environment 

Pleiotropy: a technical term for multiple effects of multiple genes.

Dirt contains bacteria, especially mycobacteria, which stimulate one part of the immune system, whereas routine vaccination stimulates a different part of the immune system.

Our immune systems are set up in such a way that they ‘expect’ to be educated by soil mycobacteria early in childhood; when they are not, the result is an unbalanced system prone to allergy.

Arguably, more damage has been done by false negatives (true genes that have been prematurely ruled out on inadequate data) than by false positives (suspicions of a link that later prove unfounded).

Simplicity piled upon simplicity creates complexity. 

Simple determinism, whether of the genetic or environmental kind, is a depressing prospect for those with a fondness for free will.

Chromosome 6 – Intelligence 

Genes are not there to cause diseases. Most genes are not ‘broken’ in any of us, they just come in different flavours. The blue-eyed gene is not a broken version of the brown-eyed gene, or the red-haired gene a broken version of the brown-haired gene. They are different alleles – alternative versions of the same genetic ‘paragraph’, all equally fit, valid and legitimate. They are all normal; there is no single definition of normality.

Science is supposed to advance by erecting hypotheses and testing them by seeking to falsify them.

The heritability of IQ is a hypothesis that has been tested on two sets of people: twins and adoptees. The results, however you look at them, are startling. No study of the causes of intelligence has failed to find a substantial heritability.

The conclusion that all these studies converge upon is that about half of your IQ was inherited, and less than a fifth was due to the environment you shared with your siblings – the family. The rest came from the womb, the school and outside influences such as peer groups. 

But even this is misleading. Not only does your IQ change with age, but so does its heritability. The heritability of childhood IQ is about forty-five per cent, whereas in late adolescence it rises to seventy-five per cent. As you grow up, you gradually express your own innate intelligence and leave behind the influences stamped on you by others. You select the environments that suit your innate tendencies, rather than adjusting your innate tendencies to the environments you find yourself in. 

This proves two vital things: 

• Genetic influences are not frozen at conception.
• Environmental influences are not inexorably cumulative. 
• Heritability does not mean immutability.

The environment that a child experiences is as much a consequence of the child’s genes as it is of external factors: the child seeks out and creates his or her own environment. If she is of a mechanical bent, she practises mechanical skills; if a bookworm, she seeks out books. The genes may create an appetite, not an aptitude. 

Chromosome 7 – Instinct 

Conventional wisdom in 20th century social sciences:

• Everything is the product of free will, giant brains and brainwashing parents.
• Human beings do not have to rely on instinct; they learn instead; they are creative, cultural, conscious creatures. 
• Determinism in social sciences:
  • Parental determinism of Freud
  • Socio-economic determinism of Marx
  • Political determinism of Lenin
  • Peer-pressure cultural determinism of Franz Boas and Margaret Mead
  • Stimulus—response determinism of John Watson and B. F. Skinner
  • Linguistic determinism of Edward Sapir and Benjamin Whorf

For nearly a century social scientists managed to persuade thinkers of many kinds that biological causality was determinism while environmental causality preserved free will; and that animals had instincts, but human beings did not.

Chromosome X and Y – Conflict 

Females gradually evolve so that they are turned off, not on, by the displays of males of their own species. Sexual selection is thus an expression of sexual antagonism between genes for seduction and genes for resistance.

The idea of genes in conflict with each other, the notion of the genome as a sort of battlefield between parental genes and childhood genes, or between male genes and female genes, is a little-known story outside a small group of evolutionary biologists. Yet it has profoundly shaken the philosophical foundations of biology.

Chromosome 9 – Disease

Blood groups can give us insights into the history of human migrations and since 1990 they have found an entirely new role: they promise understanding of how and why our genes are all so different. They hold the key to human polymorphism. 

The first and best known of the blood group systems is the ABO system:

• O is known as the universal donor.
• A and B are ‘co-dominant’ versions of the same gene, O being the ‘recessive’ form of it.
• The difference between the A gene and the B gene is seven letters out of 1,062, of which three are synonymous or silent: that is, they make no difference to the amino acid chosen in the protein chain.
• The O group has just a single spelling change compared with A, but instead of a substitution of one letter for another, it is a deletion. In people with type O blood, the 258th letter, which should read ‘G’, is missing altogether. The effect of this is far-reaching, because it causes what is known as a reading-shift or frame-shift mutation, which is far more consequential.

The advantage will always lie with the rare version of the gene, so neither version can become extinct because if it becomes rare, it comes back into fashion. This is known, in the trade, as frequency-dependent selection, and it seems to be one of the commonest reasons that we are all so genetically diverse.

People with type O blood seem to be slightly more resistant to malaria than people of other blood groups. They also seem to be slightly less likely to get cancers of various kinds. This enhanced survival was probably enough to keep the O version of the gene from disappearing, despite its association with susceptibility to cholera. A rough balance was struck between the three variations on the blood group gene.

As they scourged our ancestors, the great epidemic diseases of the past – plague, measles, smallpox, typhus, influenza, syphilis, typhoid, chicken pox, and others – left behind their imprint on our genes. Mutations which granted resistance thrived, but that resistance often came at a price varying from severe (sickle-cell anaemia) to theoretical (the inability to receive transfusions of the wrong type of blood).

Men and women most prefer (or least dislike) the body odour of members of the opposite sex who are most different from them genetically.

Chromosome 10 – Stress 

Cholesterol is an essential ingredient of the body that is soluble in fat but not in water. It lies at the centre of an intricate system of biochemistry and genetics that integrates the whole body. The body manufactures most of its cholesterol from sugars in the diet, and could not survive without it. From cholesterol at least five crucial hormones are made, each with a very different task: progesterone, aldosterone, cortisol, testosterone and oestradiol. Collectively, they are known as the steroids. 

There is a gene on chromosome 10 called CYP17. It makes an enzyme, which enables the body to convert cholesterol into cortisol, testosterone and oestradiol. Without the enzyme, the pathway is blocked and the only hormones that can be made from cholesterol are progesterone and corticosterone. People who lack a working copy of this gene cannot make other sex hormones so they fail to go through puberty; if genetically male, they look like girls. 

Consider the other hormone that is made using CYP17: cortisol. Cortisol is used in virtually every system in the body, a hormone that literally integrates the body and the mind by altering the configuration of the brain. Cortisol interferes with the immune system, changes the sensitivity of the ears, nose and eyes, and alters various bodily functions. When you have a lot of cortisol coursing through your veins, you are – by definition – under stress. Cortisol and stress are virtually synonymous.

Short-term stressors cause an immediate increase in epinephrine and norepinephrine, the hormones that make the heart beat faster, the feet go cold. These hormones prepare the body for ‘fight or flight’ in an emergency. Stressors that last for longer activate a different pathway that results in a much slower, but more persistent increase in cortisol. One of cortisol’s most surprising effects is that it suppresses the working of the immune system.

The main purpose of most genes in the human genome is regulating the expression of other genes in the genome.

Genes need to be switched on, and external events – or free-willed behaviour – can switch on genes. Far from us lying at the mercy of our omnipotent genes, it is often our genes that lie at the mercy of us.

Cortisol levels rise in response not to the amount of work you do, but to the degree to which you are ordered about by other people.

Testosterone is just as good at suppressing the immune system as cortisol. This explains why, in many species, males catch more diseases and have higher mortality than females.

Chromosome 11 – Personality 

Dopamine pathways do many things, including controlling the flow of blood through the brain. 

• A shortage of dopamine in the brain causes an indecisive and frozen personality, unable to initiate even the body’s own movement. In the extreme form, this is known as Parkinson’s disease. 
• An excess of dopamine in the brain, by contrast, makes a mouse highly exploratory and adventurous. In human beings, excessive dopamine may be the immediate cause of schizophrenia; and some hallucinogenic drugs work by stimulating the dopamine system.

Dopamine is perhaps the brain’s motivation chemical:

• Too little and the person lacks initiative and motivation. 
• Too much and the person is easily bored and frequently seeks new adventures. 
• Here perhaps lies the root of a difference in personality.

The discovery that personality has a strong genetic component can be used in some very non-genetic therapy. The right kind of parenting can alter an innate personality. Curiously, understanding that it is innate seems to help to cure it. Telling people they are naturally shy helped them overcome that shyness. 

Marriage counsellors, too, report good results from encouraging their clients to accept that they cannot change their partners’ irritating habits – because they are probably innate – but must find ways to live with them.

Dopamine and norepinephrine are so-called monoamines. Their close cousin, another monoamine found in the brain, is serotonin, which is also a chemical manifestation of personality. But serotonin is more complicated than dopamine and norepinephrine. It is remarkably hard to pin down its characteristics. If you have unusually high levels of serotonin in your brain you will probably be a compulsive person, given to tidiness and caution, even to the point of being neurotic about it. People with the pathological condition known as obsessive-compulsive disorder can usually alleviate their symptoms by lowering their serotonin levels. At the other end of the spectrum, people with unusually low serotonin levels in their brains tend to be impulsive. Those who commit impulsive violent crimes, or suicide, are often those with less serotonin.

Melatonin is made from serotonin, so serotonin levels drop as it gets used up in melatonin manufacture. 

• The quickest way to raise serotonin levels again is to send more tryptophan into the brain, because serotonin is made from tryptophan. 
• The quickest way to send more tryptophan into the brain is to secrete insulin from the pancreas, because insulin causes the body to absorb other chemicals similar to tryptophan, thus removing competitors for the channels that take tryptophan into the brain. 
• And the quickest way to secrete insulin is to eat a carbohydrate snack.

You can alter your serotonin levels by altering your eating habits.

Serotonin levels are not innate and inflexible. They are themselves the product of social status. The higher your self-esteem and social rank relative to those around you, the higher your serotonin level is.

The whole serotonin system is about biological determinism. Your chances of becoming a criminal are affected by your brain chemistry. But that does not mean, as it is usually assumed to mean, that your behaviour is socially immutable. Quite the reverse: your brain chemistry is determined by the social signals to which you are exposed. Biology determines behaviour yet is determined by society.

Social influences upon behaviour work through the switching on and off of genes.

Chromosome 12 – Self-Assembly 

The analogy between a homeobox and a plug is quite close: the homeobox is the bit by which the protein made by the gene attaches to a strand of DNA to switch on or off another gene. All homeotic genes are genes for switching other genes on or off.

From the basic asymmetry of chemicals injected into the egg all else follows:

• Genes turn each other on, giving the embryo a head and a rear. 
• Other genes then get turned on in sequence from bow to stern giving each compartment an identity. 
• Other genes then polarise the compartments into front and rear halves. 
• Other genes then interpret all this information and make ever more complicated appendages and organs. 
• It is a rather basic, chemical—mechanical, step-by-step process. From simple asymmetry can grow intricate pattern. 

So simple is embryonic development in principle – though not in detail — that it is tempting to wonder if human engineers should not try to copy it, and invent self-assembling machines.

Chromosome 13 – Pre-History 

As late as the eighteenth century in Europe, the rich drank nothing but wine, beer, coffee and tea. They risked death otherwise.

The gene for lactase: this enzyme is necessary for the digestion of lactose, a sugar abundant in milk. We are all born with this gene switched on in our digestive system, but in most mammals – and therefore in most people — it switches off during infancy. This makes sense: milk is something you drink in infancy and it is a waste of energy making the enzyme after that.

Evidence suggests that such people took up a pastoral way of life first, and developed milk-digesting ability later in response to it. It was not the case that they took up a pastoral way of life because they found themselves genetically equipped for it. This is a significant discovery. It provides an example of a cultural change leading to an evolutionary, biological change. The genes can be induced to change by voluntary, free-willed, conscious action. By taking up the sensible lifestyle of dairy herdsmen, human beings created their own evolutionary pressures.

Chromosome 14 – Immortality

Telomere: its presence enables the DNA-copying devices to get started without cutting short any sense-containing ‘text’. Like an aglet, the little plastic bit on the end of a shoelace, it stops the end of the chromosome from fraying.

Every time the chromosome is copied, a little bit of the telomere is left off. After a few hundred copyings, the chromosome is getting so short at the end that meaningful genes are in danger of being left off. In your body the telomeres are shortening at the rate of about thirty-one ‘letters’ a year – more in some tissues. That is why cells grow old and cease to thrive beyond a certain age.

The reason that genes do not get left off in egg cells and sperm cells, the direct ancestors of the next generation, is the presence of telomerase, whose job is to repair the frayed ends of chromosomes, re-lengthening the telomeres.

The prime risk factor for cancer is age. Environmental risk factors, such as cigarette smoking, work in part because they accelerate the ageing process: they damage the lungs, which require repair and repair uses up telomere length, thus making the cells ‘older’ in telomere terms than they would otherwise be. Tissues that are especially prone to cancer tend to be tissues that do a lot of cell division throughout life either for repair or for other reasons: skin, testis, breast, colon, stomach, white blood cells.

We have a paradox: shortened telomeres mean higher cancer risk, but telomerase, which keeps telomeres long, is necessary for a tumour. The resolution lies in the fact that the switching on of telomerase is one of the essential mutations that must occur if a cancer is to turn malignant.

Chromosome 15 – Sex 

Most of the striatum, cortex and hippocampus of the mouse brain are consistently made by these maternal cells, but that such cells are excluded from the hypothalamus. The cortex is the place where sensory information is processed and behaviour is produced. 

Paternal cells, by contrast, are comparatively scarce in the brain, but much commoner in the muscles. Where they do appear in the brain, however, they contribute to the development of the hypothalamus, amygdala and preoptic area. These areas comprise part of the ‘limbic system’ and are responsible for the control of emotions.

The evidence from zoology has always pointed that way: male behaviour is systematically different from female behaviour in most species and the difference has an innate component. The brain is an organ with innate gender. 

Chromosome 16 – Memory 

The human genome is a book. By reading it carefully from beginning to end, taking due account of anomalies like imprinting, a skilful technician could make a complete human body.

Instinct is genetically-determined behaviour; learning is behaviour modified by experience.

“The main function of consciousness is to enable [the child] to learn things which natural heredity fails to transmit.” – Baldwin

Since the process of natural selection is one of extracting useful information from the environment and encoding it in the genes, there is a sense in which you can look on the human genome as four billion years’ worth of accumulated learning.

The Baldwin effect is about the delicate balance between cultural and genetic evolution. They are not opposites, but comrades, trading influence with each other to get best results.

Sea slugs, in other words, are capable of the same kinds of learning as dogs or people: habituation, sensitisation and associative learning. Yet they do not even use their brains. These reflexes and the learning that modifies them occur in the abdominal ganglion, a small nervous substation in the belly of the slimy creature.

What is learning? What changes occur to nerve cells when the brain (or the abdominal ganglion) acquires a new habit or a change in its behaviour? 

• The central nervous system consists of lots of nerve cells, down each of which electrical signals travel; and synapses, which are junctions between nerve cells. 
• When an electrical nerve signal reaches a synapse, it must transfer to a chemical signal, like a train passenger catching a ferry across a sea channel, before resuming its electrical journey. 
• Kandel’s attention quickly focused on these synapses between neurons. 
• Learning seems to be a change in their properties. 
• Thus when a sea slug habituates to a false alarm, the synapse between the receiving, sensory neuron and the neuron that moves the gill is somehow weakened. 
• Conversely, when the sea slug is sensitised to the stimulus, the synapse is strengthened. 
• Gradually and ingeniously, Kandel and his colleagues homed in on a particular molecule in the sea-slug brain which lay at the heart of this weakening or strengthening of the synapses. The molecule is called cyclic AMP.

Intelligence requires a judicious mixture of remembering and forgetting.

The fact that the volado gene‘s job is to make a protein that binds cells together raises the intriguing hint that memory may consist, quite literally, of the tightening of the connections between neurons. When you learn something, you alter the physical network of your brain so as to create new, tight connections where there were none or weaker ones before.

Chromosome 17 – Death 

Tumour suppressors are the opposite of oncogenes. Whereas oncogenes cause cancer if they are jammed on, tumour-suppressor genes cause cancer if they are jammed off.

Another sign of trouble that alerts p53 is if the cell starts to run short of oxygen, which is a diagnostic feature of tumour cells. Inside a growing ball of cancer cells, the blood supply can run short, so the cells begin to suffocate. Malignant cancers get over this problem by sending out a signal to the body to grow new arteries into the tumour – the characteristic, crab-claw-like arteries that first gave cancer its Greek name.

TP53 seems to encode the greater good, like a suicide pill in the mouth of a soldier that dissolves only when it detects evidence that he is about to mutiny. The suicide of cells in this way is known as apoptosis, from the Greek for the fall of autumn leaves. It is the most important of the body’s weapons against cancer, the last line of defence. Indeed, so important is apoptosis that it is gradually becoming clear that almost all therapeutic cancer treatment works only because it induces apoptosis by alerting p53 and its colleagues.

Not only does the apoptotic cull of neurons enable learning to take place, it also improves the average quality of the cells that remain. Something similar probably happens in the immune cells, another subject to ruthless culling of cells by apoptosis.

Apoptosis is a decentralised business. That is the beauty of it. Like the development of the embryo, it harnesses the self-knowledge of each cell. There is only one conceptual difficulty: how apoptosis could have evolved. 

• In passing the test of killing itself if infected, cancerous or genetically mischievous, a cell by definition dies. It cannot therefore pass on its goodness to its daughters. 
• Known as ‘the kamikaze conundrum‘, this problem is solved by a form of group selection: whole bodies in which apoptosis works well do better than whole bodies in which it fails to work; the former therefore pass on the right traits to the cells of their offspring. 
• But it does mean that the apoptotic system cannot improve during a person’s lifetime, because it cannot evolve by natural selection within the body. We are stuck with the cell-suicide mechanism that we inherited.

Chromosome 18 – Cures 

A retrovirus contains a message written in RNA which reads: ‘Make a copy of me and stitch it into your chromosome.’ All a gene therapist need do is take a retrovirus, cut out a few of its genes (especially those that make it infectious after the first insertion), put in a human gene, and infect the patient with it. The virus goes to work inserting the gene into the cells of the body and you have a genetically modified person.

Far from being unsafe, gene therapy seemed more likely to be unworkable. Each retrovirus can only infect one kind of tissue; it needs careful packaging to get the genes into its envelope; it lands at random anywhere among the chromosomes and often fails to get switched on; and the body’s immune system, primed by the crack troops of infectious disease, does not miss a clumsy, home-made retrovirus.

Transgenic mice are scientific gold dust. They enable scientists to find out what genes are for and why. The inserted gene need not be derived from a mouse, but could be from a person: unlike in computers, virtually all biological bodies can run any kind of software. For instance, a mouse that is abnormally susceptible to cancer can be made normal again by the introduction of a human chromosome 18.

Chromosome 19 – Prevention 

Heart disease is a preventable and treatable condition. Those with the E2 gene in particular are acutely sensitive to fatty and cholesterol-rich diets and are thus easily treated by being warned off such diets. This is extremely valuable genetic knowledge. How many lives could be saved, and early heart attacks averted, by simple genetic diagnosis to identify those at risk and target treatment at them?

Chromosome 20 – Politics 

There are plenty of inherited diseases, and contagious diseases in which inheritance determines susceptibility — cholera being a now classic case — but the notion that an infectious particle could somehow travel through the germline seemed to break all the rules of biology.

Chromosome 21 – Eugenics 

Down-syndrome babies are generally born to older mothers. The probability of having a Down-syndrome baby grows rapidly and exponentially as the age of the mother increases, from 1 in 2,300 at the age of twenty to 1 in 100 at forty.

Chromosome 22 – Free Will 

Hume’s fork: Either our actions are determined, in which case we are not responsible for them, or they are the result of random events, in which case we are not responsible for them.

We are spectacularly resistant to brainwashing. No matter how hard their parents or their politicians tell them that smoking is bad for them, young people still take it up. Indeed, it is precisely because grown-ups lecture them about it that it seems so appealing. We are genetically endowed with a tendency to be bloody-minded towards authority, especially in our teens, to guard our own innate character against dictators, teachers, abusing step-parents or government advertising campaigns.

There was new, strong evidence against what Rich Harris calls ‘the nurture assumption’. Studies of the divorce rate of twins, for example, reveal that genetics accounts for about half of the variation in divorce rate, non-shared environmental factors for another half and shared home environment for nothing at all. In other words, you are no more likely to divorce if reared in a broken home than the average – unless your biological parents divorced.

It is now clear that children probably have more non-genetic effect on parents than vice versa.

People get their personalities from their genes and from their peers, not from their parents.

‘The opposite of freedom is coercion, not determinism.’

Far from loving free will, we seem to be a species that positively leaps to surrender it whenever we can. Full responsibility for one’s actions is a necessary fiction without which the law would flounder, but it is a fiction all the same. To the extent that you act in character you are responsible for your actions; yet acting in character is merely expressing the many determinisms that caused your character.

Pierre-Simon de LaPlace once mused that if, as a good Newtonian, he could know the positions and the motions of every atom in the universe, he could predict the future. Or rather, he suspected that he could not know the future, but he wondered why not.

Chaos theory provides a better answer to LaPlace. Unlike quantum physics, it does not rest on chance. Chaotic systems, as defined by mathematicians, are determined, not random. But the theory holds that even if you know all the determining factors in a system, you may not be able to predict the course it will take, because of the way different causes can interact with each other. Even simply determined systems can behave chaotically. They do so partly because of reflexivity, whereby one action affects the starting conditions of the next action, so small effects become larger causes. The trajectory of the stock market index, the future of the weather and the ‘fractal geometry’ of a coastline are all chaotic systems: in each case, the broad outline or course of events is predictable, but the precise details are not.

Human behaviour shares these characteristics. Stress can alter the expression of genes, which can affect the response to stress and so on. Human behaviour is therefore unpredictable in the short term, but broadly predictable in the long term. Thus at any instant in the day, I can choose not to consume a meal. I am free not to eat. But over the course of the day it is almost a certainty that I will eat.

Freedom lies in expressing your own determinism, not somebody else’s. It is not the determinism that makes a difference, but the ownership.

A gene for free will would not be such a paradox because it would locate the source of our behaviour inside us, where others cannot get at it. Of course, there is no single gene, but instead there is something infinitely more uplifting and magnificent: a whole human nature, flexibly preordained in our chromosomes and idiosyncratic to each of us. Everybody has a unique and different, endogenous nature. A self.