“It runs in the family” gets said about almost everything: height, temperament, a talent for music, a tendency toward certain health conditions. It’s such a common phrase that it’s worth asking a more basic question underneath all of it. How do scientists actually determine whether something is genetic, rather than simply a pattern that looks that way because families share households, habits, and culture along with DNA?
The answer involves a handful of specific research methods, each designed to separate genetic influence from environmental influence as cleanly as possible. Understanding how these methods work makes it much easier to judge which family patterns have real scientific backing, and which are simply common assumptions that haven’t been tested carefully.
Contents
- Twin Studies: The Classic Method for Measuring Heritability
- Adoption Studies: Separating Genes From Shared Environment
- What a “Heritability Percentage” Actually Means, and Doesn’t
- From Candidate Genes to Genome-Wide Studies: How the Science Improved
- Epigenetics: When Environment Changes How Genes Are Expressed
- Finding These Markers in Your Own Raw DNA File
- Frequently Asked Questions
Twin Studies: The Classic Method for Measuring Heritability
The most well-established method for estimating how genetic a trait is involves comparing identical twins, who share essentially all of their DNA, to fraternal twins, who share roughly half, the same as typical siblings. If identical twins consistently show more similarity in a given trait than fraternal twins do, that’s strong evidence the trait has a real genetic component, since both types of twins are usually raised in similarly shared environments.
This comparison produces what researchers call a heritability estimate, typically expressed as a percentage. Twin studies have been running for decades and across huge numbers of participants, making this one of the more reliable tools in behavioral and medical genetics, even though it comes with real limitations, including the assumption that identical and fraternal twins experience similarly shared environments, which isn’t always perfectly true.
A second major method involves studying adopted children, comparing their traits to both their biological relatives, who share genetics but not environment, and their adoptive relatives, who share environment but not genetics. When adopted children resemble their biological relatives more closely on a specific trait, despite growing up in a completely different household, that’s compelling evidence the trait has a meaningful genetic basis.
Adoption studies and twin studies tend to be used together, since each method has different strengths and limitations. When both approaches point to a similar heritability estimate for the same trait, researchers treat that agreement as considerably stronger evidence than either method would provide alone.
What a “Heritability Percentage” Actually Means, and Doesn’t
Heritability estimates get reported often, but they’re widely misunderstood. A heritability estimate of 50 percent doesn’t mean half of an individual person’s trait comes from genetics and half from environment. It means that, across a population, roughly half of the variation between individuals in that trait is statistically associated with genetic differences, with the remaining variation associated with environmental differences. It’s a population-level statistic, not a personal formula, and it can shift depending on how much environmental variation exists within the population being studied.
This distinction matters because heritability percentages are often casually applied to individuals in ways the statistic was never designed to support. A trait with a heritability estimate of 40 percent doesn’t mean any specific person is “40 percent genetic” in that trait. It describes a pattern across a group, not a formula for any one person.
From Candidate Genes to Genome-Wide Studies: How the Science Improved
Earlier genetic research often focused on candidate genes, testing one or two genes suspected of influencing a trait based on a plausible biological story. This approach produced some real findings, but also a fair number of results that didn’t hold up under later scrutiny, since small sample sizes made it easier to find patterns that turned out to be coincidental.
Modern research increasingly relies on genome-wide association studies, commonly called GWAS, which scan hundreds of thousands of genetic variants simultaneously across very large populations, sometimes hundreds of thousands of participants. This approach has produced a more reliable, if often more modest, picture of genetic influence, generally finding that most traits are affected by many genes each contributing a small effect, rather than one or two genes with a large effect.
Epigenetics: When Environment Changes How Genes Are Expressed
A newer area of research, epigenetics, adds another layer to this picture. Epigenetics studies how environmental factors, including diet, stress, and other life experiences, can affect whether certain genes are actively expressed, without changing the underlying DNA sequence itself. This helps explain why identical genetic variants can sometimes produce different outcomes in different people, and it’s an active area of ongoing research rather than a fully settled science.
Finding These Markers in Your Own Raw DNA File
The genetic variants studied through twin studies, adoption research, and genome-wide association studies are part of the same broad panel read during standard ancestry testing, meaning relevant markers already exist in the raw DNA file downloaded from AncestryDNA, 23andMe, MyHeritage, or FamilyTreeDNA, even though ancestry platforms don’t report on them. SelfDecode, a genetics and health analysis platform, allows that existing file to be uploaded directly, generating organized reports drawn from this same body of research across areas like metabolism, inflammation, and mood.
An uploaded file only provides a limited preview of this analysis. Because it was originally generated by a different company’s lab using different chip technology, it may not include every marker SelfDecode’s system reads, and the resulting report is narrower than what a sample processed directly through SelfDecode’s own lab would provide.
For a fuller picture, including a broader set of reports beyond what an uploaded file offers, the SelfDecode At-Home DNA Test Kit, priced at approximately $99, processes a new sample through SelfDecode’s own lab from the start.
“It runs in the family” turns out to be a phrase with real scientific machinery behind it, built from decades of twin studies, adoption research, and increasingly large genetic datasets, even if the full answer is usually more nuanced than the phrase itself lets on.
Frequently Asked Questions
How do scientists know if a trait is genetic?
Researchers primarily use twin studies, comparing identical to fraternal twins, and adoption studies, comparing adopted children to both biological and adoptive relatives, to estimate how much genetics contributes to a given trait.
What does a heritability percentage actually mean?
It’s a population-level statistic describing how much of the variation between individuals in a trait is associated with genetic differences. It doesn’t mean any single person’s trait is a fixed percentage genetic and the rest environmental.
What is a GWAS study?
GWAS stands for genome-wide association study. It scans hundreds of thousands of genetic variants across very large populations simultaneously, producing more reliable findings than older studies that focused on just one or two candidate genes at a time.
What is epigenetics?
Epigenetics studies how environmental factors can affect whether certain genes are actively expressed, without changing the underlying DNA sequence. It helps explain why the same genetic variant can produce different outcomes in different people.
Are older candidate-gene studies still considered reliable?
Some findings from candidate-gene studies have held up, but many have not replicated well in larger, more rigorous studies. Modern genome-wide research is generally considered more reliable for estimating genetic influence.
