A copy of Jeffrey Craig’s gene sequence sits on his laptop, a file mapping variations of genes unique to him. The professor has the sequence for safekeeping; it once helped him learn that a blood clot he suffered after a flight wasn’t from an inherited condition. But as for what else it might predict, for now he doesn’t want to know. “It’s a Pandora’s box,” Craig says. “If you get your gene sequence you need to be thinking about, really, what do you want to know?”

Genetic testing is becoming increasingly powerful. It can pinpoint whether someone has a rare disease or whether they carry a gene that could result in their offspring having a life-threatening condition. It is also now being used to score someone’s risk of contracting more common ailments such as heart disease or depression. Some people use the tests to inform their choices about lifestyle or starting a family.

Meanwhile, the traits we inherit, for better or worse, are becoming clearer with genetic testing too. Our genes never fully determine who we are, but it seems the aptitudes and characteristics we love to attribute to them – the singing ability we share with our sister, the maths brain we “inherited” from our mother, the gift of the gab our grandpa was famous for – are indeed influenced by our biology.

So, what can and can’t our genes really tell us about ourselves? Is how we act – as well as how healthy we are – coded in our DNA?

What are our genes, genome and DNA?

When a sperm and egg meet they form a single cell, tying together DNA from each parent. As the cell multiplies trillions of times to make up a human body, each new cell contains a complete copy of our DNA, tiny and tightly coiled. Magnified a million times, DNA is arranged on a double helix, two long strings that twist. “The sequence of that string carries information that your cells can use to make things,” says Professor Daniel MacArthur, director of the Centre for Population Genomics at the Garvan Institute of Medical Research and Murdoch Children’s Research Institute. Along this string are 3.2 billion locations where information is set out.

Our genome is the entire information sequence on the string. Despite its microscopic size, if unwound it would extend for about two metres – life’s biological recipe hanging by a thread. In all of our cells, this string is divided into stretches called chromosomes – most of us carry 23 of these inherited from our mother, and a matching set inherited from our father – wound tightly into compact, orderly structures that keep it safe and stop the string from tangling.

Genes make up yet smaller sections of the string. Many genes are simply involved in regulating cells, but 23,000 lay down instructions for cells to make proteins. “Proteins are mostly the doing molecules in the cell,” MacArthur says. “They do things such as helping your cells move, or make energy, or clean toxins from your blood.”

Here are some examples of how particular genes, located in certain chromosomes, can affect us.

There’s yet another layer to the complexity. Although every cell has a full copy of our DNA, our cells receive chemical signals from their environment or other cells, using the information to switch genes on and off so they perform the right functions. This happens before birth: “The way that genes are switched on and off is what determines whether a particular cell becomes a bone cell or a muscle cell,” says MacArthur. But it also happens throughout our lives. As our skin cells constantly turn over, for example, genes turn on keratins to toughen the skin and protect us against injury.

Lifestyle can cause genes to be switched on and off, or expressed, in different ways. As identical twins who share almost all of their DNA are exposed to the sun, toxins, stress or changes in diet, for instance, they could end up appearing differently. “DNA [is] like a recipe. It lays out some instructions but the way those instructions get interpreted and become a human being will differ depending on the environment,” says MacArthur.

How much do our genes vary?

Humans are far more genetically alike than we are different. The genes between any two people are 99.9 per cent the same. There are similarities beyond our kind too: we share more than 98 per cent of our DNA with chimpanzees. Even a mouse has certain types of genes that are 85 per cent alike to humans.

But if any two people are 99.9 per cent similar to one another, what of the remaining .1 per cent? These genetic variations make each of us unique, influencing our physical characteristics and health and, to some extent, our talents. In producing each of us, our parents were capable of having trillions of genetically unique offspring. “There’s never been another human who’s had your exact DNA sequence and there never will be,” MacArthur says. “There’s more possible combinations of genetic variation than there are atoms in the universe.” (Even in identical twins, small numbers of genetic changes have occurred to each during the first few cell divisions when they were an embryo.)

More than 12,000 variations influence height, yet many add less than a centimetre to someone’s stature.

Just a few genetic variations determine the colour of our eyes, hair and skin but more often, hundreds or thousands are involved. More than 12,000 influence height, for example, yet many add less than a centimetre to someone’s stature. “If you have many of these different kinds of [genetic variations], each contributing even just a few millimetres, then the combination of them can make you 10 centimetres or 20 centimetres taller,” says forensic genetics Professor Dennis McNevin of the University of Technology Sydney. Even the most minor differences among people – whether their earlobe joins at their head, or the shape of their hairline – are influenced by many genes.

McNevin has processed DNA samples of suspects for police investigations and helped identify bodies. His testing can reveal a person’s sex, whether their ancestors are from any of eight broad regions, and their hair and eye colour. “It becomes too difficult and it’s too inaccurate after that to predict other traits,” McNevin says, although he foresees “the holy grail” of predicting someone’s face might be feasible one day.

Not all genetic variations are traits we’re born with. You might have heard that certain variations influence diseases such as cancer. As humans grow and age, cells turn over. “We’re always making new cells and replacing the ones that we’ve gotten rid of,” MacArthur says. Every time a cell replicates, it makes a near-perfect copy of our DNA. “It’s astonishing that this process happens so accurately.”

Errors can occur, usually benign, but a genetic change that falls in the wrong place could lead to cancer. “It can result in a cell that no longer knows how to stop dividing,” MacArthur says.

What can our genes tell us about our health?

When genetic research was first emerging, scientists wanted to find single genes that influenced diseases. They found several thousand single-gene (monogenic) disorders, which cause rare conditions such as muscular dystrophy. But for more common diseases, such as cancer and heart attacks, they found multiple genes (polygenic) influencing a person’s risk.

Several genetic variations that cause disease are so dominant it takes just one parent to pass them on. Children of a parent with the mutation for Huntington’s disease, for instance – which gradually shuts down the brain – have a 50 per cent chance of inheriting the condition.

‘This isn’t Gattaca. It’s providing parents with information about ... [the] risk of really awful diseases.’

While it’s well known that blue eyes or red hair are passed down in recessive genes – traits that skip generations – we can also carry a genetic disease without knowing it. We have two copies of every gene, inheriting one from each parent, which gives many damaged genes a back-up. But if our partner has the same genetic error, our offspring can inherit both damaged genes, causing conditions such as cystic fibrosis. “These are what we call silent diseases that can just suddenly appear apparently out of nowhere,” MacArthur says. “[They] can pass down within a family for many generations without anyone ever actually being sick.” Would-be parents can screen for their risk of passing on serious conditions. “This isn’t Gattaca,” MacArthur says of the science-fiction film about genetic perfectionism. “It’s providing future parents with information about whether the children may be at risk of really awful diseases.”

In diseases involving multiple genes, particular variations can cause more havoc than others. The BRCA1 and BRCA2 mutations, for example, can put women at up to 70 per cent risk of developing breast cancer and increase their risk of ovarian cancer. Similarly, changes in several genes involved in cholesterol put women at a 30 per cent risk of having a heart attack by age 60, and men at a 50 per cent risk by age 50. Identifying genetic risks can encourage susceptible people to screen for diseases more regularly, to make lifestyle changes and to receive medication or preventative surgery earlier.

What can genes tell us about our behaviour?

Four weeks after entering the world in 1939, identical twins James Springer and James Lewis were adopted into separate Ohio homes. They lived in different towns until reuniting 39 years later. What they discovered still stuns Nancy Segal, a developmental psychology professor at California State University, who met the twins twice. They both bit their nails, suffered similar headaches and had worked part-time in law enforcement and at McDonald’s. Their first wives’ names were Linda, they remarried women named Betty, and they named their first sons James. They even holidayed within blocks of each other in Florida. “It’s amazing they didn’t meet over the years,” Segal says.

A coincidence, surely? Perhaps in some instances, Segal says, but too many similarities popped up to ignore. The Jims inspired a landmark Minnesota study of 137 twins reared apart, which Segal was involved in for a decade, finding shared traits across intelligence, preferred leisure activities and social attitudes. The surprises kept coming: some twins liked unusual Swedish brands of toothpaste or had picked out the same bedspreads. In Australia, twin sisters unknown to one another worked at David Jones and Myer, respectively, in the 1990s before colleagues joined the dots and reunited them. Segal recounts in her book Born Together-Reared Apart their similar jewellery, clothes and choice of cigarette brand.

Fifty years of twin studies have landed on genes influencing just shy of half of any given trait.

She says chance can’t explain why some twins who’ve never met can share rare traits. “The fact [the Jim twins] had the same vacation spot; it’s unusual,” Segal tells us. “When you see these things in identical twins raised apart [who share almost all their DNA], it tells you something, especially when you don’t see them in fraternal twins raised apart or together.”

Unlike with some physical traits, genes do not share links as strongly with behaviour. Still, twins offer a unique opportunity for researchers to peel apart nature and nurture, and when they do it’s hard to find a trait where genetics isn’t at play. According to 50 years of twin studies, mostly in those raised together, genes influence just shy of half of any given trait. Our attitudes towards religion and politics, desire to play sport, or even our zest for life fit this trend, says Segal. “We showed in our study that interests have a genetic component; interests in reading, even twins who had interests and never did anything [with it], both said they always wanted to be an actor.”

Most people can’t tell which sister is singing when identical twins Paula and Lee Bowman perform in their country rock band, Jetty Road. As pre-schoolers, they would sing the national anthem together before watching Sesame Street. In high school, their ability to harmonise stunned a music teacher so much they made a trio and began touring country Victoria. “We could always hold a tune to start off with,” Lee says. “We have these instincts when we sing that are the same ... it feels like an extension of us.”

They were among more than 500 twins in a study published in 2022 that concluded genetics wields a 40 per cent influence over singing talent. Using an online tool, twins from all around Australia recorded themselves singing Happy Birthday, then hitting a single note and picking up an unfamiliar tune. The study measured how well they reached notes and intervals.

Performance, of course, didn’t just come down to genetics. Shared experiences with music, such as families singing together in the car, influenced 37 per cent of the twins’ talent. Then maybe one twin took music lessons and the other didn’t – differences the remaining 23 per cent accounts for. Having both nature and nurture “sets you up for success”, says Sarah Wilson, a professor of clinical neuropsychology at the University of Melbourne, who led the study.

Wilson has also exmained how singing passes through families, comparing ability across three generations. “What we’re seeing is that it’s being expressed at every generation,” she says. “We can follow it through the family tree.” The Bowmans fondly recall singing with their father. But as far as they know, the talent didn’t extend to older generations; Paula suspects if her relatives had singing talent, they didn’t receive the encouragement and opportunities she and Lee had. “Our talent, from a young age, was seen.” One of the few people who can differentiate the Bowmans’ voices is Paula’s daughter, Jasmine. “She has exactly what I had,” Paula says of the seven-year-old’s ear for music. “We’ll see what happens in a few years.”

Is IQ influenced by genes?

Nick Martin was at uni in Adelaide in the early 1970s when he conducted his first study into intelligence in twins. He used the state’s school exams roll, contacting everyone with the same surname, birthdate and address. Identical twins logged far more similar academic results than non-identical twins, suggesting genetics were at play. “I thought, wow, that’s right on your doorstep.”

Martin, now a behavioural geneticist at QIMR Berghofer Medical Research Institute, says IQ is up to 80 per cent genetically influenced, even taking into account education and socioeconomic status. Separate studies have found genetics has a 20 per cent influence on IQ during infancy and increases as we age. “IQ is the main driver of cognitive ability. But you can get genes that influence specialisation in maths, literature, or art or music,” Martin says. “These abilities, including say football ability, do often run in families at a much higher probability than one would expect by chance, but ... we can also find striking contrasts within families.”

From Martin’s perspective, education policies that assume everyone is equally malleable are unhelpful, and he advocates for non-academic pathways to have equal standing. “I think the most important thing is that kids should be brought up with some sense of fulfilment with whatever they do.”

Jeffrey Craig, who is the deputy director at Twins Research Australia, says outcomes of IQ are particularly difficult to separate from socioeconomic status. “People have the genetics they are born with, but if they don’t get support they may not reach their full potential.” He also points out that IQ is just one of many abilities, alongside general and emotional intelligence. Craig and Martin agree that how genetics affects behaviour can be framed in a troubling, deterministic manner. “It starts with an author writing a paper using the word determines or predicts; I hate both of those words. Influences is fine,” Craig says.

What can genes show about mental health? Martin now scans hundreds of thousands of genetic variations across huge databases, comparing trends in what are called genome-wide association studies. The results show the many genes that together influence mental health. He found a high genetic association for schizophrenia, for example, and around 37 per cent for depression. Martin says awareness that genes can affect mental health can be helpful. Opportunities for people to test can help them take steps, such as exercising or nurturing friendships, as a first line of defence.

What can’t genetic testing tell us?

Most genetic tests in Australian clinics and hospitals, usually done via a blood test or a saliva sample, are monogenic – screening for single genes that influence a disease. They can help diagnose diseases, screen for whether someone is carrying a recessive gene, and test whether a fetus has certain genetic disorders. Tatiane Yanes, a genetic counsellor at the Queensland Children’s Hospital, uses the metaphor of a jar and pebbles when explaining results to patients and families. Someone’s genetic risk of skin cancer, for example, can add only so many pebbles to the jar. Their exposure to ultraviolet light and lack of sun protection can add more pebbles, causing the jar to overflow. “It’s not clear-cut,” she says of test results. “There is some interpretation that goes on.”

When many genes play a role in a disease, such as heart disease or depression, scientists use a polygenic risk score, which adds up thousands of genetic variations and calculates the risk of someone developing certain diseases over their lifespan. The technology is in its early stages and has not been adopted as standard practice in Australian healthcare; tests are available directly to consumers through commercial companies. “These aren’t magic but they do seem to provide, for a substantial proportion of the population, information that can be used,” MacArthur says.

Yanes, who at the University of Queensland researches genetic testing rollouts, says results with polygenic testing can differ depending on which provider a sample goes to. “That’s one of the big concerns; we have no standard around how this is done,” she says. She also stresses genetics in some conditions add only “a small number of pebbles to the jar”. “You can pretty much develop [a risk score] for any condition because everything has somewhat of a genetic component. It doesn’t mean you should, or it has clinical utility.”

Genetic testing has other limitations too. In 2021, 86 per cent of genomic research studies worldwide were in individuals of European descent. Polygenic scores calculated in Europeans don’t transfer neatly to people of different backgrounds. MacArthur says as genetic testing becomes more accurate in the next few years, it also needs to become more diverse or “we run a real risk of some communities being left behind”.

Meanwhile, faced with the option, some people want as much genetic information as possible. MacArthur and his wife screened themselves before having their three sons, and his wife tested during the pregnancies. “Our view was that we were better off having that information and being able to make an informed decision,” he says. Other people find too much information stressful, especially without the kind of context that counsellors in hospital settings can provide. “We can’t be paternalistic about this,” MacArthur says. “It should be about choice; people should decide what they want to know.”

Jeffrey Craig has considered whether looking further into the genome results on his laptop would help him live a better life. He’s concerned it could show that he or loved ones are at risk of conditions such as Alzheimer’s or Parkinson’s. While he could use the information to possibly take precautions, he fears feeling trapped by it. “To me, it’s more important that you look at general fitness. It’s something that you can change,” he says. “There’s no point finding out you could get something unless something can help you.”

He is also intrigued by randomness and the unknown.As he studies twins, he marvels at their earliest differences in size, placentas and umbilical cords, and that they’ll forge unique lives. “It’s really twins who have taught me about those differences,” he says. “We will never fully be able to predict what they are.”

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