When the heart breaks down, it beats erratically or not at all. A bone can chip or snap. But when the complex network of neurons in our brain malfunctions, the result can be a near-endless variety and combinations of mental illnesses.

It’s normal to sometimes be sad, happy, anxious, confused, forgetful or fearful, but when a person’s emotions, thoughts or behaviour frequently trouble them, or disrupt their lives, they may be suffering from mental illness. According to the World Health Organization (WHO), about 450 million people worldwide are affected by mental, neurological or behavioural problems at any time.

However, determining that someone has a mental illness, and which one, is one of the challenges psychiatrists face. One effort to catalogue these afflictions is the “psychiatrists’ bible”, the Diagnostic and Statistical Manual of Mental Disorders – the latest edition fills nearly one thousand pages and lists over 400 disorders.

Diversity of disorders

Among the best known and most common mental illnesses is depression – a prolonged, debilitating sadness, sometimes accompanied by a feeling of hopelessness and thoughts of suicide. Seasonal affective disorder is a type of depression that affects some people in the autumn and winter and is triggered by the shrinking hours of daylight and colder temperatures. In bipolar disorder, a person swings from depression to episodes of mania where they are euphoric, energetic and unrealistically confident in their abilities.

Personality disorders are behaviour patterns that are destructive to the person themselves or those around them. In dissociative disorders, someone experiences a sudden change in consciousness or their concept of self. In dissociative amnesia, for example, the result is a loss of part or all of their memories. Samson, the Biblical strongman, may have suffered from the earliest recorded case of antisocial personality disorder.

Anxiety disorders are characterised by powerful feelings of stress and physical signs of fear – sweating, a racing heart – due to some cue in the environment, or for no obvious reason at all. These include post-traumatic stress disorder, panic disorder, obsessive compulsive disorder, anger disorders, hypochondria, social phobia, and other phobias including agoraphobia (open spaces), claustrophobia (small spaces), acrophobia (heights), and arachnophobia (spiders).

Enormous cost

Eating disorders involve an unhealthy relationship to food. A sufferer of anorexia nervosa will strive for thinness to the point of starvation, due to a distorted perception of their body and dissatisfaction in their sense of control. People with bulimia engage in cycles of gorging themselves and then purging through vomiting or use of laxatives. Muscle dysmorphia is sometimes thought of as a “reverse” form of anorexia that affects bodybuilders. Sufferers constantly worry that they are too puny, despite being extremely muscular.

Attention-deficit hyperactivity disorder is among the most common mental illnesses diagnosed in children, affecting their ability to focus and associated with high levels of activity and impulsiveness.

Mental illnesses are quite common. As many as one in five people are thought to suffer from mental illness, at least temporarily, each year. Suicide – often the result of untreated mental illness – claims 873,000 lives around the world each year. The economic costs of these conditions are also enormous and growing. According to the WHO, depression is expected to account for more lost years of healthy life than any other disease by 2030, except for HIV/AIDS.

Even so, the mentally ill face stigma and discrimination. Studies find people are reluctant to admit they have a mental illness, to seek help, or to stick with treatment. Others are eager to reject the label of a mental illness. For example, some people with autism – characterised by difficulty communicating or socialising – insist the condition is not a disorder that needs to be cured, but just part of normal human “neurodiversity”.

Underlying causes

Historically, some symptoms of mental illness, such as erratic behaviour and hearing voices, have been taken as evidence of heavenly communication or demonic possession.

More recently, brain scans have directly linked these conditions with changes in levels of neurotransmitters – chemicals that convey messages across neurons – or alterations in the number or structure of neurons in different brain areas. For instance, people suffering from depression often display lowered levels of the neurotransmitter serotonin.

In a few cases, the immediate cause of the malfunction has been identified. Alzheimer’s disease, a major source of dementia and memory loss in the elderly, is caused by the accumulation of protein plaques which choke neurons in the brain.

Some infectious diseases can also develop into a mental illness. Untreated HIV infection can cause dementia, as can the uncontrolled replication of the microbe that causes syphilis. Borrelia burgdorferi – the Lyme disease bacterium, the Borna disease virus and the toxoplasma parasite, responsible for malaria, are also thought capable of triggering a variety of mental illnesses.

In many cases the precise cause is unclear and experts suspect that many different factors are involved. One striking example is schizophrenia, distinguished by psychosis. This is a distorted view of reality, which may include hallucinations, hearing voices, delusions, and paranoia. The chance that identical twins both develop schizophrenia is much higher than that for fraternal twins or siblings, arguing for the strong role of inherited genes. But scientists are accumulating a growing list of other risk factors that predispose people to this condition, including prenatal exposure to famine conditions, certain infections or exposure to lead. The season of their birth also seems important – birth in winter or early spring increases the risk, as does an older father and, controversially, child abuse.

Genes are also thought to influence many other mental health problems, including: anorexia, autism, Alzheimer’s disease and bipolar disorder.

Some other factors that have been linked to mental illness include the womb environment, exposure to X-rays, being held in detention centres and having an overactive immune system.

Some researchers believe that smoking cigarettes and taking recreational drugs like LSD, ecstasy and cannabis, may elevate a user’s risk of mental illnesses, including schizophrenia – although it can be difficult to assess whether drug use is a cause or effect. And careful use of LSD and ecstasy might even help treat psychiatric problems.

Psychiatric treatments

Psychiatric treatment for mental illness can take many forms. In psychotherapy, the patient is encouraged to recognise their problems, understand what may trigger undesirable behaviour, and develop coping strategies.

Many medications are also available to treat some of the most severe symptoms. Mood-stabilising drugs aim to moderate manic episodes of bipolar disorder and may also reduce recurrences of depression. Antipsychotics reduce the reality-bending symptoms of schizophrenia. Anti-depressants include drugs like Prozac – known as selective serotonin reuptake inhibitors or SSRIs – which slow the removal of serotonin in the brain, thus increasing the neurotransmitter‘s availability.

Recently, however, some experts think there has been a rush to medicate every disorder and have questioned the effectiveness of many drugs. There is also controversy about using these drugs – such as Ritalin or amphetamines – to treat children.

Other less mainstream treatments for mental health problems, include stimulating the brain with magnetic pulses, electroconvulsive therapy, deep brain electrode stimulation, staying at a Hindu temple and using virtual reality to treat schizophrenia and phobias. Some experts argue that the different treatments for depression share a common mechanism – prompting the growth of neurons.

Madly creative

Madness has long been linked with genius. Many famous artists, writers and scientists have suffered from mental disorders, leading some to wonder if there is a link between these illnesses and creativity.

The mathematician John Nash struggled with schizophrenia while he developed the theory that earned him a Nobel Prize. The artist Vincent Van Gogh, the composer Robert Schumann and the writer Fyodor Dostoevsky are said to have suffered from a range of mental disorders including hypergraphia, a compulsion to write – a sign perhaps their art emerged from an unrelenting urge to communicate.

One possibility is that genes that predispose people to such devastating illnesses persist because when the syndromes are present in a milder form, this heightened creativity gives people an evolutionary advantage.

1. What is DNA?

DNA is the storehouse of genetic information for every known organism, with the exception of a few viruses. It’s a long, thin molecule – picture two strands that curve around each other, forming a double helix. Each strand spells out the genetic code as a chain of four chemical letters called bases: adenine (A), thymine (T), cytosine (C) and guanine (G).

Bases facing each other across two strands are always paired as follows: A with T and C with G. When cells duplicate their DNA, the two strands of the helix are “unzipped” and enzymes use them as a template to create a new version of the opposite strand.

2. What is a gene?

This is not a simple question.

For decades, biologists like the late Francis Crick – co-discoverer of DNA’s structure – confidently proclaimed that genes were regions of DNA that served as blueprints for proteins through a simple process: DNA is copied into the related chemical RNA, which then is whisked away to the cell’s protein manufacturing facility. Crick famously dubbed this definition of the gene the “central dogma of molecular biology”.

A gene’s protein sequence is spelled out as a series of three letter “words” – codons – composed of the four DNA bases. The codon “GGG” for instance, encodes the amino acid glycine.

Regions of DNA that do not produce proteins were therefore generally dismissed as functionless “junk DNA”. But it turns out that Crick’s dogma may have been a bit too, well, dogmatic.

Insights from the sequencing of the human genome have led some experts to argue that the purpose of many human genes is not to encode protein, but to spin out RNAs that serve many functions beyond that of middleman between DNA and protein. And the evolutionary conservation of some types of junk DNA suggests they serve important, if unknown, functions.

3. How do genes create organisms?

Genes are not always active. Sometimes they are busy churning out their encoded protein or RNA, sometimes they are shut off completely. And their activity can be tuned at different levels. During the development of a complex organism from a single cell, thousands of genes flash on and off in complicated patterns.

One of the most important jobs genes have is to create proteins called transcription factors, which coordinate the activities of other genes. As an eye or a finger is created, for example, transcription factors ensure that a characteristic series of genes get activated in surrounding tissue to build that structure.

Proteins and structures they create can also serve many other functions: generating energy, creating new molecules or serving directly as the brick and mortar of structures like muscle. Genes also shape organisms by driving the replication, movement, activity and death of cells.

4. What can go wrong with the process?

The genetic code is so precise that even a change in a single DNA base can have profound effects. The mutation which causes the disease sickle cell anaemia, for example, was tracked down to the substitution of a T for an A in the gene for the protein haemoglobin, with carries oxygen in red blood cells. As a result, a single protein building block called an amino acid is changed, resulting in a crippled protein.

Sometimes the problem is not the gene sequence, but the location or number of genes. Whole regions of chromosomes can be missing or duplicated, resulting in missing genes or inappropriate activity. Cancer cells, for instance, often have the wrong number of entire chromosomes.

5. How are genes inherited?

Our genes and the 23 pairs of chromosomes they reside on are inherited, with one of each pair coming from each parent. This means that sperm and eggs must contain half the number of chromosomes of any other cell in the body. Otherwise when sperm and egg fused to form an embryo it would contain twice the number of chromosomes needed.

Sperm and eggs receive their half-portion of genes through a chromosomal choreography called meiosis. First the 23 parental pairs of chromosomes match up at the centre of the sex cell. When the cell divides, each daughter receives only one half of each pair. Since this process is random, it generates a staggering 70 trillion possible combinations of chromosomes in the offspring.

In fact, the true degree of possible variation is higher because the maternal and paternal chromosomes exchange DNA when they pair, creating new gene combinations within the chromosomes. So rest assured of your genetic uniqueness – unless you are an identical twin.

6. What other factors control how our genes work?

It seems reasonable that if two genes with the same sequence are in the same cell, they should act the same way. But that is not always true. So-called epigenetic factors can alter how a gene works regardless of its DNA sequence.

One well studied example is parental imprinting. Certain genes are marked with chemical tags via a process called methylation while they are still in a sperm or egg, meaning that only the maternal or paternal copy is active in the offspring. As a result, certain traits are inherited exclusively from one side of the family.

There is also some evidence that environment can influence epigenetic factors. For example, Dutch women who were pregnant during the famines of the World War II gave birth to small babies. But, surprisingly, the next generation also spawned small babies even though they ate well, as if they “inherited” their mother’s experience.

7. Do genes control everything about an organism, or is environment important?

The debate over the relative importance of nature and nurture would fill several encyclopedias, but modern genetics predicts that both should play a role. That should come as no surprise to anyone who views genes as a piece of cellular machinery. Dangerous chemicals, such as cigarette smoke, can jam that machinery or interfere in its workings.

Equally, a therapeutic environment can compensate for a faulty gene. For example, babies who are born with the disease phenylketonuria (PKU) lack an enzyme that metabolises the amino acid phenylalanine. It therefore builds up to toxic levels causing mental retardation. But babies are now screened for the defect at birth and those with two copies of the defective gene are given special diets low in phenylalanine. As a result, they develop normally.

8. How genetically similar are we to primates and other organisms?

Chimpanzee genes differ, on average, by roughly just 1% from human genes. Other apes’ genes are 95% to 98% identical to ours, too. Rodent genes are 88% identical and chickens come in at 75% identical.

Once you leave the animal kingdom, wholesale comparisons between human genes and those of other species becomes trickier. About one-third of the genome of the fruit fly Drosophila melanogaster, for example, contains genes that are only shared by other arthropods, and one-quarter of human genes are shared only by vertebrates.

The function of some genes in flies, plants or worms appear close enough to their human counterparts that these animals can serve as models to study human biology and disease.

9. Most traits are controlled by a complex array of genes. But which human features depend entirely on single genes?

You already know some of your single gene traits like the back of your hand. Specific versions of different single genes cause: hair growth on the middle segments of the fingers, the top of the little finger to bend dramatically to the ring finger, and determine whether the left thumb crosses over the right – or vice versa – when fingers are interlocked.

The result of other single gene traits are as plain as the eyes, ears, and hair on your face. People with blue eyes, non-dangly ear lobes or a straight hairline have inherited specific gene varieties. The ability to roll your tongue into a tube and also to taste certain bitter chemicals is also conferred by certain types of single genes.

Defective versions of single genes can also cause disease such as cystic fibrosis, sickle cell anaemia and Huntington’s disease.

No field of science has changed more, or changed the world more, in the last 50 years than genetics – the study of how our physical and behavioural traits are inherited.

The field’s crowning achievement may have been the spelling out of our genetic secrets by the human genome project, but scientific and technological advances in genetics have forever transformed agriculture, biology, medicine, zoology, and even fields such as anthropology and forensic science.

Why certain features of parents and even more distant relatives appear or do not appear in individual people, plants, parasites and protozoa has fascinated and confused people for millennia. This observation has also spawned a remarkable variety of theories of heredity, from pangenesis to Lamarckism. Modern genetics, however, can trace its lineage to pea plants in the garden of an Augustinian monk, Gregor Mendel. By studying the inheritance of traits such as plant height and wrinkly peas, he discovered that most hereditary traits are carried by discrete factors, later called genes.

Dominant and recessive

These experiments illuminated many of the key principles of genetics. For example, they revealed that most organisms have two copies of each gene, one from each parent, and that a gene comes in a variety of different forms, or alleles.

A purebred tall pea plant has two tall alleles of a gene for height (often abbreviated as TT), and the short plants have two short alleles (tt). Their offspring have one of each (Tt). This first generation is tall because the tall allele is dominant.

Recessive traits, such as shortness in pea plants, are only expressed when two recessive alleles meet up. This is also a good example of how organisms of the same appearance, or phenotype (the tall parent and their offspring), can have different genotypes, or combinations of genes (TT versus Tt).

It was nearly 100 years after Mendel published his work that scientists discovered genes are composed of the double-helical molecule DNA, which is built from four chemical letters, or bases: adenine, thymine, cytosine and guanine. The discovery of the structure of DNA in 1953 immediately suggested a simple mechanism for DNA replication: the two strands of the helix could unzip and allow enzymes to enter and synthesise two new strands.

The goal of the human genome project was to use DNA sequencing to reveal all three billion DNA letters in our chromosomes and find all our genes. By comparing our genetic make-up to the genomes of mice, chimps and a menagerie of other species (rats, chickens, dogs, pufferfish, the microscopic worm Caenorhabditis elegans, the fruit fly Drosophila melanogaster and many bacteria), scientists have learned a great deal about how genes evolve over time, and gained insights into human diseases.

Another powerful technology leading the genetics revolution is the polymerase chain reaction (PCR), which allows large quantities of DNA sequence information to be derived from tiny and highly damaged samples.

PCR has become the linchpin in many criminal investigations, now that traces of blood, semen or skin left at a crime scene (even decades previously) can condemn a criminal or exonerate innocent suspects. This technology is also reshaping notions of both human ancestry and the evolutionary history of many species, by harvesting genes from ancient remains, including fossil DNA. Other new techniques could prove even faster than PCR.

Life’s instruction manual

Here’s how genes work. When a gene is switched on, one of its strands is copied into the related chemical RNA, a process known as transcription. For most known genes this “messenger” or mRNA is then shuttled off to a ribosome of a cell where its translation into a protein sequence occurs. Ribosomes decipher the sequence of an mRNA with the help of small cloverleaf shaped molecules called “transfer” or tRNA.

Some mRNAs also contain introns, extra sequences that do not encode protein and must be spliced out before they journey to the ribosome. The protein-coding portions of these mRNAs are called exons.

In inherited diseases, some part of this process has gone awry due to a mutation, or change in the DNA sequence. Cystic fibrosis, achondroplasia, phenylketonuria and Huntington’s disease are each caused by simple changes in the sequence of a single gene.

Other diseases with a more complex genetic component include diabetes and epilepsy. Down’s syndrome, haemophilia, fragile X syndrome, sickle-cell anaemia, thalassaemia and a plethora of other disorders are also down to genetic defects.

The runaway replication of cancer cells can also traced to genetic causes – mutations that remove the normal controls on cell growth.

In our cells, many hundreds of genes are joined end-to-end on DNA strands and tightly packed into sausage-shaped structures called chromosomes which are stored in a cell’s nucleus. A locus is the position of a gene on a chromosome. Chromosomes also possess physical features which protect their genetic cargo. Telomeres at their ends act as caps to prevent biochemical wear and tear, while the cell choreographs the movement of chromosomes by grabbing hold of its centromere handle.

The dance of chromosomes is tightly controlled during cell replication to preserve the genetic makeup of cells. During normal growth, cells replicate through mitosis and each “daughter” inherits the same number of chromosomes and genes as its parent. But sperm and eggs contain half the number of chromosomes because they form through a different process called meiosis. Then when sperm and egg fuse to make a zygote, it contains the same number of genes and chromosomes as its parents.

Sex and linkage

Many human genes are inherited exactly as Mendel envisaged, with one copy from each parent. Human blood type falls into this category of Mendelian inheritance. But heredity can also be more complicated. DNA sequences that lie near each other on chromosomes exhibit gene linkage and are usually inherited together, even though they may be unrelated in function. Another exception are so-called sex-linked traits, encoded on the sex chromosomes.

Men inherit one X and one Y chromosome (which encodes the gene for maleness), so they are more likely to express recessive traits from the X, such as colour blindness and haemophilia. Women are more protected, because they have two X chromosomes. Finally, the mitochondria – the energy powerhouse of the cell – contains its own small parcel of mitochondrial DNA which is inherited exclusively from the egg, and therefore the mother.

The genetic variation of individual DNA sequences in a population is called its genetic diversity. Diversity results from the re-assortment of genes during meiosis and genetic mutation. Genetic variation drives evolution by creating a range of phenotypes that might give individuals a competitive edge in different environments.

Those individuals with the fittest combination of alleles produce more offspring. Inbreeding can be dangerous for a population because it removes variation and the ability to adapt to new environments. While natural selection favours the accumulation of fit alleles of beneficial genes, the majority of chromosomes in many organisms are composed of “selfish DNA“, which does not benefit its host and seems to play no other role other than ensuring its own replication.

Genetic engineering

The most powerful aspect of the new genetics is genetic engineering – the ability to design new genetic sequences and insert them into viruses, bacteria, flies, plants, mice and other animals. This has been a boon for research, and led to the birth of biotechnology and many practical applications.

Doctors have been experimenting with gene therapy to supply healthy copies of a gene to patients suffering from a genetic defect. Genetically engineered vaccines incorporate genetic pieces of a deadly microbe to stimulate the immune system, without the danger of infection. And companies are using genetic modification to create hardier plants and animals that resist disease, grow faster or produce healthier food.

But critics warn that genetic engineering has a dark side too, and could lead to designer babies or the release of dangerous DNA sequences into the gene pool, with dire environmental consequences.

Diversity of disorders

Enormous cost

Underlying causes

Madly creative

1. What is DNA?

2. What is a gene?

3. How do genes create organisms?

4. What can go wrong with the process?

5. How are genes inherited?

6. What other factors control how our genes work?

7. Do genes control everything about an organism, or is environment important?

8. How genetically similar are we to primates and other organisms?

9. Most traits are controlled by a complex array of genes. But which human features depend entirely on single genes?

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1. What is DNA?

DNA is the storehouse of genetic information for every known organism, with the exception of a few viruses. It’s a long, thin molecule – picture two strands that curve around each other, forming a double helix. Each strand spells out the genetic code as a chain of four chemical letters called bases: adenine (A), thymine (T), cytosine (C) and guanine (G).

Bases facing each other across two strands are always paired as follows: A with T and C with G. When cells duplicate their DNA, the two strands of the helix are “unzipped” and enzymes use them as a template to create a new version of the opposite strand.

Back to top

2. What is a gene?

This is not a simple question.

For decades, biologists like the late Francis Crick – co-discoverer of DNA’s structure – confidently proclaimed that genes were regions of DNA that served as blueprints for proteins through a simple process: DNA is copied into the related chemical RNA, which then is whisked away to the cell’s protein manufacturing facility. Crick famously dubbed this definition of the gene the “central dogma of molecular biology”.

A gene’s protein sequence is spelled out as a series of three letter “words” – codons – composed of the four DNA bases. The codon “GGG” for instance, encodes the amino acid glycine.

Regions of DNA that do not produce proteins were therefore generally dismissed as functionless “junk DNA”. But it turns out that Crick’s dogma may have been a bit too, well, dogmatic.

Insights from the sequencing of the human genome have led some experts to argue that the purpose of many human genes is not to encode protein, but to spin out RNAs that serve many functions beyond that of middleman between DNA and protein. And the evolutionary conservation of some types of junk DNA suggests they serve important, if unknown, functions.

Back to top

3. How do genes create organisms?

Genes are not always active. Sometimes they are busy churning out their encoded protein or RNA, sometimes they are shut off completely. And their activity can be tuned at different levels. During the development of a complex organism from a single cell, thousands of genes flash on and off in complicated patterns.

One of the most important jobs genes have is to create proteins called transcription factors, which coordinate the activities of other genes. As an eye or a finger is created, for example, transcription factors ensure that a characteristic series of genes get activated in surrounding tissue to build that structure.

Proteins and structures they create can also serve many other functions: generating energy, creating new molecules or serving directly as the brick and mortar of structures like muscle. Genes also shape organisms by driving the replication, movement, activity and death of cells.

Back to top

4. What can go wrong with the process?

The genetic code is so precise that even a change in a single DNA base can have profound effects. The mutation which causes the disease sickle cell anaemia, for example, was tracked down to the substitution of a T for an A in the gene for the protein haemoglobin, with carries oxygen in red blood cells. As a result, a single protein building block called an amino acid is changed, resulting in a crippled protein.

Sometimes the problem is not the gene sequence, but the location or number of genes. Whole regions of chromosomes can be missing or duplicated, resulting in missing genes or inappropriate activity. Cancer cells, for instance, often have the wrong number of entire chromosomes.

Back to top

5. How are genes inherited?

Our genes and the 23 pairs of chromosomes they reside on are inherited, with one of each pair coming from each parent. This means that sperm and eggs must contain half the number of chromosomes of any other cell in the body. Otherwise when sperm and egg fused to form an embryo it would contain twice the number of chromosomes needed.

Sperm and eggs receive their half-portion of genes through a chromosomal choreography called meiosis. First the 23 parental pairs of chromosomes match up at the centre of the sex cell. When the cell divides, each daughter receives only one half of each pair. Since this process is random, it generates a staggering 70 trillion possible combinations of chromosomes in the offspring.

In fact, the true degree of possible variation is higher because the maternal and paternal chromosomes exchange DNA when they pair, creating new gene combinations within the chromosomes. So rest assured of your genetic uniqueness – unless you are an identical twin.

Back to top

6. What other factors control how our genes work?

It seems reasonable that if two genes with the same sequence are in the same cell, they should act the same way. But that is not always true. So-called epigenetic factors can alter how a gene works regardless of its DNA sequence.

One well studied example is parental imprinting. Certain genes are marked with chemical tags via a process called methylation while they are still in a sperm or egg, meaning that only the maternal or paternal copy is active in the offspring. As a result, certain traits are inherited exclusively from one side of the family.

There is also some evidence that environment can influence epigenetic factors. For example, Dutch women who were pregnant during the famines of the World War II gave birth to small babies. But, surprisingly, the next generation also spawned small babies even though they ate well, as if they “inherited” their mother’s experience.

Back to top

7. Do genes control everything about an organism, or is environment important?

The debate over the relative importance of nature and nurture would fill several encyclopedias, but modern genetics predicts that both should play a role. That should come as no surprise to anyone who views genes as a piece of cellular machinery. Dangerous chemicals, such as cigarette smoke, can jam that machinery or interfere in its workings.

Equally, a therapeutic environment can compensate for a faulty gene. For example, babies who are born with the disease phenylketonuria (PKU) lack an enzyme that metabolises the amino acid phenylalanine. It therefore builds up to toxic levels causing mental retardation. But babies are now screened for the defect at birth and those with two copies of the defective gene are given special diets low in phenylalanine. As a result, they develop normally.

Back to top

8. How genetically similar are we to primates and other organisms?

Chimpanzee genes differ, on average, by roughly just 1% from human genes. Other apes’ genes are 95% to 98% identical to ours, too. Rodent genes are 88% identical and chickens come in at 75% identical.

Once you leave the animal kingdom, wholesale comparisons between human genes and those of other species becomes trickier. About one-third of the genome of the fruit fly Drosophila melanogaster, for example, contains genes that are only shared by other arthropods, and one-quarter of human genes are shared only by vertebrates.

The function of some genes in flies, plants or worms appear close enough to their human counterparts that these animals can serve as models to study human biology and disease.

Back to top

9. Most traits are controlled by a complex array of genes. But which human features depend entirely on single genes?

You already know some of your single gene traits like the back of your hand. Specific versions of different single genes cause: hair growth on the middle segments of the fingers, the top of the little finger to bend dramatically to the ring finger, and determine whether the left thumb crosses over the right – or vice versa – when fingers are interlocked.

The result of other single gene traits are as plain as the eyes, ears, and hair on your face. People with blue eyes, non-dangly ear lobes or a straight hairline have inherited specific gene varieties. The ability to roll your tongue into a tube and also to taste certain bitter chemicals is also conferred by certain types of single genes.

Defective versions of single genes can also cause disease such as cystic fibrosis, sickle cell anaemia and Huntington’s disease.

Back to top

Philip Cohen, 3 November 2005

The field’s crowning achievement may have been the spelling out of our genetic secrets by the human genome project, but scientific and technological advances in genetics have forever transformed agriculture, biology, medicine, zoology, and even fields such as anthropology and forensic science.

Why certain features of parents and even more distant relatives appear or do not appear in individual people, plants, parasites and protozoa has fascinated and confused people for millennia. This observation has also spawned a remarkable variety of theories of heredity, from pangenesis to Lamarckism. Modern genetics, however, can trace its lineage to pea plants in the garden of an Augustinian monk, Gregor Mendel. By studying the inheritance of traits such as plant height and wrinkly peas, he discovered that most hereditary traits are carried by discrete factors, later called genes.

Dominant and recessive

These experiments illuminated many of the key principles of genetics. For example, they revealed that most organisms have two copies of each gene, one from each parent, and that a gene comes in a variety of different forms, or alleles.

A purebred tall pea plant has two tall alleles of a gene for height (often abbreviated as TT), and the short plants have two short alleles (tt). Their offspring have one of each (Tt). This first generation is tall because the tall allele is dominant.

Recessive traits, such as shortness in pea plants, are only expressed when two recessive alleles meet up. This is also a good example of how organisms of the same appearance, or phenotype (the tall parent and their offspring), can have different genotypes, or combinations of genes (TT versus Tt).

It was nearly 100 years after Mendel published his work that scientists discovered genes are composed of the double-helical molecule DNA, which is built from four chemical letters, or bases: adenine, thymine, cytosine and guanine. The discovery of the structure of DNA in 1953 immediately suggested a simple mechanism for DNA replication: the two strands of the helix could unzip and allow enzymes to enter and synthesise two new strands.

The goal of the human genome project was to use DNA sequencing to reveal all three billion DNA letters in our chromosomes and find all our genes. By comparing our genetic make-up to the genomes of mice, chimps and a menagerie of other species (rats, chickens, dogs, pufferfish, the microscopic worm Caenorhabditis elegans, the fruit fly Drosophila melanogaster and many bacteria), scientists have learned a great deal about how genes evolve over time, and gained insights into human diseases.

Another powerful technology leading the genetics revolution is the polymerase chain reaction (PCR), which allows large quantities of DNA sequence information to be derived from tiny and highly damaged samples.

PCR has become the linchpin in many criminal investigations, now that traces of blood, semen or skin left at a crime scene (even decades previously) can condemn a criminal or exonerate innocent suspects. This technology is also reshaping notions of both human ancestry and the evolutionary history of many species, by harvesting genes from ancient remains, including fossil DNA. Other new techniques could prove even faster than PCR.

Life’s instruction manual

Here’s how genes work. When a gene is switched on, one of its strands is copied into the related chemical RNA, a process known as transcription. For most known genes this “messenger” or mRNA is then shuttled off to a ribosome of a cell where its translation into a protein sequence occurs. Ribosomes decipher the sequence of an mRNA with the help of small cloverleaf shaped molecules called “transfer” or tRNA.

Some mRNAs also contain introns, extra sequences that do not encode protein and must be spliced out before they journey to the ribosome. The protein-coding portions of these mRNAs are called exons.

In inherited diseases, some part of this process has gone awry due to a mutation, or change in the DNA sequence. Cystic fibrosis, achondroplasia, phenylketonuria and Huntington’s disease are each caused by simple changes in the sequence of a single gene.

Other diseases with a more complex genetic component include diabetes and epilepsy. Down’s syndrome, haemophilia, fragile X syndrome, sickle-cell anaemia, thalassaemia and a plethora of other disorders are also down to genetic defects.

The runaway replication of cancer cells can also traced to genetic causes – mutations that remove the normal controls on cell growth.

In our cells, many hundreds of genes are joined end-to-end on DNA strands and tightly packed into sausage-shaped structures called chromosomes which are stored in a cell’s nucleus. A locus is the position of a gene on a chromosome. Chromosomes also possess physical features which protect their genetic cargo. Telomeres at their ends act as caps to prevent biochemical wear and tear, while the cell choreographs the movement of chromosomes by grabbing hold of its centromere handle.

The dance of chromosomes is tightly controlled during cell replication to preserve the genetic makeup of cells. During normal growth, cells replicate through mitosis and each “daughter” inherits the same number of chromosomes and genes as its parent. But sperm and eggs contain half the number of chromosomes because they form through a different process called meiosis. Then when sperm and egg fuse to make a zygote, it contains the same number of genes and chromosomes as its parents.

Sex and linkage

Many human genes are inherited exactly as Mendel envisaged, with one copy from each parent. Human blood type falls into this category of Mendelian inheritance. But heredity can also be more complicated. DNA sequences that lie near each other on chromosomes exhibit gene linkage and are usually inherited together, even though they may be unrelated in function. Another exception are so-called sex-linked traits, encoded on the sex chromosomes.

Men inherit one X and one Y chromosome (which encodes the gene for maleness), so they are more likely to express recessive traits from the X, such as colour blindness and haemophilia. Women are more protected, because they have two X chromosomes. Finally, the mitochondria – the energy powerhouse of the cell – contains its own small parcel of mitochondrial DNA which is inherited exclusively from the egg, and therefore the mother.

The genetic variation of individual DNA sequences in a population is called its genetic diversity. Diversity results from the re-assortment of genes during meiosis and genetic mutation. Genetic variation drives evolution by creating a range of phenotypes that might give individuals a competitive edge in different environments.

Those individuals with the fittest combination of alleles produce more offspring. Inbreeding can be dangerous for a population because it removes variation and the ability to adapt to new environments. While natural selection favours the accumulation of fit alleles of beneficial genes, the majority of chromosomes in many organisms are composed of “selfish DNA“, which does not benefit its host and seems to play no other role other than ensuring its own replication.

Genetic engineering

The most powerful aspect of the new genetics is genetic engineering – the ability to design new genetic sequences and insert them into viruses, bacteria, flies, plants, mice and other animals. This has been a boon for research, and led to the birth of biotechnology and many practical applications.

Doctors have been experimenting with gene therapy to supply healthy copies of a gene to patients suffering from a genetic defect. Genetically engineered vaccines incorporate genetic pieces of a deadly microbe to stimulate the immune system, without the danger of infection. And companies are using genetic modification to create hardier plants and animals that resist disease, grow faster or produce healthier food.

But critics warn that genetic engineering has a dark side too, and could lead to designer babies or the release of dangerous DNA sequences into the gene pool, with dire environmental consequences.

Philip Cohen, 3 November 2005

The rovers landed on Mars in January 2004 with solar cells capable of providing more than 900 watt-hours of electricity per day. Spirit’s output has dropped to about 400 watt-hours, partly because Martian dust has caked its solar panels.

Opportunity’s output also declined at first – to around 500 watt-hours – but over the past six months it has regained power (��è����Ƶ print edition, 30 October). Lately, its solar cells have been delivering just over 900 watt-hours.

Rover team leader Jim Erickson at NASA’s Jet Propulsion Laboratory in Pasadena, California, told ��è����Ƶ that a process still not understood has repeatedly removed dust from the solar panels. “These exciting and unexplained cleaning events have kept Opportunity in really great shape,” he says.

Whatever the process, it has taken place while Opportunity was parked during the Martian night. On at least four occasions over a six-month period, the rover’s power output increased by up to 5% overnight. At the time, the team speculated that wind may have swept the dust off the panels or frost may have caused it to clump, exposing more of the panels.

Self inspection

Now an inspection of the rover’s surface using its own camera has confirmed that dust has been removed from the vehicle. Erickson estimates that the cleaning accounts for about 15% of the difference between Spirit’s and Opportunity’s power output. Most of the remaining disparity is due to the difference in sunshine in their two locations.

But the mystery of why only Opportunity has been cleaned remains. The answer might lie in the nature of the two rovers’ missions. Spirit has been prospecting in an area called Columbia Hills, while Opportunity has been exploring the wall of Endurance crater.

While climbing, Opportunity spent a lot of time with its solar panels tilted, which could have caused any dust to tumble off. And the researchers suspect the shape of the crater may encourage the development of dust devils or other wind patterns that could help scrub the panels.

If the crater does provide a natural, wind-driven car wash then Opportunity’s days as a clean machine could be at an end. On 12 December, it drove out of the crater to explore the terrain beyond. “If in three or four months Opportunity is still operating and hasn’t had another power boost that would suggest the crater was the key,” Erickson says.