Genetic, apparently.

“It’s genetic” sounds like an answer.

For most of my life, I think I heard it that way too. If something was genetic, you were born with it. It was written somewhere inside your DNA before you had any say in the matter, and there wasn't much you could do about it. Genetics felt almost like biological fortune telling. Here are the instructions you were given. Good luck.

I still catch myself thinking that way all the time.

Something about my body looks like someone in my family? Genetics. A disease shows up in multiple relatives? Genetics. Someone is naturally tall, naturally thin, naturally athletic, naturally good at math? Probably genetics. We use the word so casually that “genetic” starts sounding interchangeable with “inevitable.”

The more genetics I learn, though, the less satisfying that answer becomes. Because there are absolutely things DNA can determine very strongly. There are diseases caused primarily by pathogenic variants in a single gene. There are recognizable Mendelian inheritance patterns where geneticists can calculate the probability of a child inheriting a particular variant. Biology does occasionally give us something close to the clean Punnett squares we learned in school. But most of the traits people casually call genetic are nowhere near that simple. Height is genetic. Intelligence has genetic influences. Obesity has genetic influences. Depression and other psychiatric conditions can have genetic influences. Type 2 diabetes, heart disease and many cancers have genetic components. Alzheimer's has genetic influences. And none of those sentences means, “There is one gene for this, and if you have it, this is what you will become.” That distinction seems small until you realize how differently it makes you think about people.

A lot of human characteristics are polygenic, meaning variation in the trait is influenced by many genes rather than one. And many are multifactorial, meaning genetic variation is only part of the story. Environment, development, exposures, behavior, socioeconomic conditions and other biological factors can matter too. Common conditions such as obesity, heart disease and type 2 diabetes are examples of complex conditions shaped by multiple genetic and environmental influences.

So when somebody says something “runs in the family,” even that deserves another question: What exactly is running through the family? Genes certainly do. But families share a lot more than DNA.

They can share food. Neighborhoods. Income. Stress. Access to healthcare. Pollutant exposures. Education. Cultural practices. Exercise habits. Expectations. Trauma. Recipes. Jobs. Sleep schedules. Ways of coping with problems. Even the language a family speaks is passed from one generation to another without being genetically inherited. Something can be familial without being entirely genetic.

Obesity is an easy place to see how messy this becomes. There are genetic variants that influence things involved in body weight and metabolism, and obesity can cluster in families. But body weight exists inside an environment too. Food availability, physical activity, medications, sleep, stress, socioeconomic circumstances and other factors can interact with biological predisposition. Most obesity isn't explained by one “obesity gene.” It is a complex trait influenced by many factors. So “obesity runs in my family” can be biologically meaningful without translating into “therefore I was genetically destined to have this exact body.” Those are completely different claims. I think intelligence is where this conversation gets even more uncomfortable because people desperately want a clean answer to something we don't even define particularly cleanly.

Do I think I'm naturally intelligent? God, no.

I actually think I struggle more than the average person. I don't experience learning as this effortless thing where information enters my brain and stays there because I was blessed with academically superior DNA. I work at it. I reread things. I have to figure things out. I become interested in something, realize I'm decent at understanding it, keep going, get better at it, and eventually that becomes something people look at and call an ability.

That doesn't mean biology has nothing to do with cognitive differences. Research does find genetic contributions to variation in cognitive traits, but intelligence is a complex trait, not the product of one “smart gene.” And even the word intelligence can hide a ridiculous number of things underneath it. Memory? Processing speed? Verbal ability? Spatial reasoning? Creativity? Problem solving? Academic performance? Curiosity? The ability to sit down and force yourself through something difficult?

I think a lot of what gets called intelligence from the outside is also opportunity and work. I think about that with my parents.

They did not have the educational opportunities I have had. So how would I ever know what their academic abilities would have looked like under completely different circumstances? Put someone into survival mode, give them responsibilities early, limit their schooling, make work more immediately necessary than education, and then decades later it would be ridiculous to point at their educational outcome and treat it like a measurement of their biological potential.

If I had my exact genome but had been born somewhere else, raised by different people, spoken another language, attended different schools, had different expectations placed on me and been exposed to completely different things, I think I would be a completely different person.

Same DNA. Different life.

That's the part of genetics I find fascinating. DNA matters enormously, but DNA doesn't develop in a vacuum. Even height, one of the traits we think of as obviously genetic, makes the point beautifully. Height is highly heritable in many studied populations, yet nutrition, childhood illness and other environmental conditions can affect how fully someone's growth potential is realized. A population can therefore have a highly heritable trait while its average expression still changes as environmental conditions change. And that brings me to one of the most abused words in genetics: heritability.

Suppose I told you a trait was 70 percent heritable. The natural interpretation is that 70 percent of the trait comes from genes and the remaining 30 percent comes from environment.

That is not what it means.

Heritability is a population statistic. It describes how much of the variation between people in a particular population, under particular environmental conditions, is associated with genetic differences among them. A heritability estimate of 0.7 does not mean that 70 percent of an individual person's intelligence, height, depression, body weight or anything else was “caused by their genes.” And because populations and environments differ, heritability estimates aren't universal constants carved into nature. I love this distinction because it completely ruins the way people casually throw genetic percentages around. You cannot divide a person into percentages like a pie chart. This much of me came from DNA. This much came from my parents. This slice is school. This little piece is trauma. This part is because I read too much.

Human development doesn't work like that because genes and environments aren't always independent forces competing over who gets credit for the final product. They interact.

A genetic variant can influence how a body responds to an environment. An environment can determine whether a genetic susceptibility ever becomes particularly important. Genes can influence behaviors that change the environments people seek out. Other genes can modify the effects of a particular variant. Sometimes even possessing a disease-associated variant isn't enough to tell us whether somebody will actually develop the disease. Genetics has a word for that too: penetrance.

Penetrance basically asks: among people who carry a particular variant, how many actually show the associated trait or disease? Some variants have incomplete or reduced penetrance, meaning some people who carry them never develop the condition. Even among people who do develop a genetic disorder, the severity and features can differ, something geneticists call variable expressivity. I think that's one of the strangest things about biology. Two people can carry the same important genetic variant and still not live the same biological story. Then there are identical twins. Identical twins are useful to genetics precisely because their genomes are extraordinarily similar. If DNA were a perfectly deterministic script for every complex human outcome, you might expect identical twins to essentially become biological copies of one another. They don't.

They can differ in disease, behavior and other traits. Their environments and life experiences aren't perfectly identical, and biological differences can accumulate over time. For complex traits, having nearly the same inherited DNA does not guarantee the same outcome. Even your own genome is not necessarily an instruction manual with one predetermined ending. Maybe it makes more sense to think of it as establishing possibilities and probabilities. That becomes especially important with disease. A genetic predisposition means that a person's genetic makeup increases susceptibility to a particular condition. It does not necessarily mean the person has the disease, nor does it guarantee they will develop it. Some risk variants have relatively small effects. Others can raise risk substantially. Multiple variants can also contribute tiny amounts of risk that add up across the genome, which is part of the idea behind polygenic risk scores.

And then, occasionally, genetics gives us something much closer to determinism.

Huntington disease is the kind of example that makes me understand why “genetic” became synonymous with destiny in the first place. A pathogenic expansion in the HTT gene has very high penetrance, and inheriting a disease-causing expansion can carry profoundly different implications than carrying one of thousands of common variants that each nudge the probability of a complex disease by a tiny amount.

Those things both belong under the enormous umbrella of genetics. They should not emotionally mean the same thing. I think determinism scares me more than probability does. Probability sucks too. Life isn't fair. Someone can take incredible care of themselves and still become sick. Another person can have risk factors stacked on top of risk factors and live a long life without developing the disease everyone expected. Biology does not distribute outcomes according to who deserves them.

Shit isn't fair. What are we going to do about it?

But there is still something about probability that leaves room. Maybe that's why I can find genetic information frightening and still want to know it. If I could see my entire genome and everything medicine currently knows how to interpret from it, I think part of me would want to. Especially if there were information relevant to something preventable, treatable or something I could potentially pass to biological children. Knowing that information could change how I lived. It could change screening decisions. It could change reproductive decisions. It could make someone more attentive to symptoms or more anxious about them.

Knowing your risk can change you before the disease ever does. I don't necessarily think that's an argument for ignorance. Information can be uncomfortable and still be useful. But I also understand why genetic testing isn't as simple as “more information is always better.” A result can sometimes give you a probability rather than an answer. Some variants have uncertain significance. Some findings have little immediate medical action attached to them. And when testing is being considered for serious inherited disease, genetic counseling can matter because the question isn't only what does this result mean biologically? It can also be what will knowing it mean for my life?

I think learning genetics has somehow made me feel both more and less in control. Less, because there are biological things happening inside us that we did not choose. Nobody chooses their genome. Nobody gets to review their variants before being born and send back the ones they don't like. There are susceptibilities we inherit, mutations that occur by chance, developmental processes we never consciously direct and diseases that can happen despite doing everything we were told was “right.” But learning genetics has also made me feel more in control because DNA is much less prophetic than I once thought. A risk is not a diagnosis. A predisposition is not a promise. Familial is not automatically genetic. Genetic is not automatically inherited. Inherited is not automatically inevitable. And highly heritable does not mean impossible to change. Those distinctions matter.

They matter when someone says depression runs in their family. Many psychiatric conditions have genetic contributions, but they are generally complex and influenced by many genetic and non-genetic factors rather than one simple inherited switch. They matter when someone talks about obesity as though it is either completely genetic or completely a matter of personal discipline. Both explanations flatten something biologically complicated. They matter when we talk about intelligence as if someone's academic performance reveals their innate biological worth. They matter when a disease appears in a family and everybody starts quietly wondering who is next. And they matter when we look at ourselves.

I used to think DNA was closer to a blueprint. That metaphor is convenient, but the more I think about it, the less I like it. A blueprint tells a builder exactly where the wall goes. A genome doesn't contain a tiny completed version of you waiting to be assembled. I think DNA is closer to a range of possibilities.

It sets constraints. It creates vulnerabilities. It contributes tendencies. Sometimes it gives extremely strong instructions. Sometimes one altered gene really can change the course of a life. Other times thousands of genetic differences each whisper something so small that their meaning only becomes visible when researchers study enormous populations. And then life happens on top of all of it.

Nutrition happens. Education happens. Stress happens. Love happens. Poverty happens. Exercise happens. Illness happens. Pollution happens. Random mutations happen. Aging happens. Other people happen. Choices happen. Things completely outside our control happen.

The genome participates in a life. It doesn't narrate the whole thing. Maybe that's why I no longer think “it's genetic” is a particularly satisfying ending to an explanation.

It's a beginning. Which genes? How much influence? In what population? Under what environment? Is this a causal variant or a risk variant? How penetrant is it? What else interacts with it? Can anything modify the outcome?

Suddenly, “it's genetic” has turned into twenty more questions.

Which, unfortunately for everyone who has ever tried to give me a simple answer, is exactly how I like things. I don't think DNA is destiny. I think it's probability. A range of possibilities. Some ranges are narrow enough to be frightening. Others are so wide that predicting one person's life from their DNA would be ridiculous.

Either way, I am not separate from my genes. They just aren't a prophecy.