Lactose Intolerance Is Mostly Genetic. Here's the Actual Mechanism

Most adults on Earth stop making enough lactase, the enzyme that breaks down milk sugar, sometime after childhood. Whether your body keeps making that enzyme into adulthood comes down largely to a single DNA switch located near a gene called LCT, on chromosome 2. That switch is a variant known as rs4988235, and whether you carry it determines if your body keeps the lactase gene turned on into adulthood.

What actually causes lactose intolerance

Lactase persistence, the trait of continuing to digest lactose as an adult, is an evolved exception rather than the default human state. The enzyme responsible, lactase-phlorizin hydrolase (LPH), is produced by the LCT gene on chromosome 2q21 and works exclusively in the small intestine, where it sits on the surface of the cells lining the gut, according to a 2020 review in the journal Nutrients by researchers at the University of Granada (NIH/PMC). In most mammals, and in most humans without the relevant variant, the LCT gene shuts off after weaning. People carrying the persistence variant just keep making the enzyme.

The variant itself doesn't sit inside the LCT gene. It's a regulatory switch about 14,000 base pairs upstream, inside an intron of a neighboring gene called MCM6. Research on genetic aspects of lactase deficiency describes this mechanism directly: lactase persistence "is determined mainly by the presence of the rs4988235-T variant in the MCM6 gene, which increases the expression of the LCT gene, encoding" the lactase enzyme, according to a 2024 paper in Vavilovskii Zhurnal Genetiki i Selektsii (NIH/PMC). The T allele keeps the LCT promoter active; the ancestral C allele lets it switch off, the way it does in most other mammals after infancy.

You'll also see this same variant referred to by an older name, -13910 C>T, which reflects its position relative to the LCT gene. A 2023 study in the International Journal of Molecular Sciences puts it plainly: "The CC genotype is a good predictor of reduced intestinal lactase expression, whereas the TT genotype is a predictor of persistently high enzymatic expression" (NIH/PMC). CT heterozygotes generally fall in between, though the practical effect on symptoms varies from person to person.

Genotype and symptoms don't always line up perfectly

Carrying two copies of the non-persistence variant doesn't automatically mean every glass of milk causes problems, and the genetic test doesn't function as a diagnosis on its own. A 2019 Mayo Clinic study of patients with irritable bowel syndrome compared the LCT-13910 genetic variant against both self-reported dairy sensitivity and hydrogen methane breath testing, precisely because those three measures don't always agree (NIH/PMC).

A 2023 study of Romanian children went further, finding that a genetic predisposition to primary lactose intolerance did not influence the children's actual dairy intake or their health-related quality of life (NIH/PMC). The same paper notes that genetic testing for LCT polymorphisms "correlates very well with positive hydrogen breath tests," which is a different claim from saying the genotype alone predicts who feels sick after a glass of milk. Some people with the non-persistent genotype tolerate moderate dairy just fine, especially in small amounts or alongside other food, while others with milder genetic risk still notice symptoms. Genetics sets the underlying enzyme capacity. What you actually feel depends on how much lactose you consume, at once, and how your gut microbiome handles what the enzyme doesn't break down.

That said, the genetic marker still has real diagnostic value. A 2026 study of a Brazilian population in the journal In Vivo concluded that testing the MCM6 gene "represents a promising diagnostic alternative for lactose intolerance, offering a feasible and less invasive approach" compared to breath testing (NIH/PMC). A DNA test won't tell you exactly how many glasses of milk you can handle, but it does tell you whether you carry the version of the gene that keeps lactase production running.

Why lactose intolerance rates differ so much by ancestry

Lactase persistence isn't evenly distributed across human populations, and that unevenness is one of the more striking examples of recent human evolution. A 2019 study from the USDA Western Human Nutrition Research Center and UC Davis notes that rs4988235 "is often used as a predictor for dairy intake, since it is responsible for [lactase persistence] in people" of certain ancestries, and the study specifically examined how genotype and ethnicity together relate to dairy consumption in a healthy US population (NIH/PMC).

The regional differences show up clearly in isolated populations that have had limited historical exposure to dairy farming. The 2024 study of indigenous Siberian populations found that lactase deficiency driven by the absence of the rs4988235-T variant was common, consistent with a population history that didn't involve widespread dairy pastoralism (NIH/PMC). Populations with a long history of cattle or goat herding, in contrast, tend to carry the persistence variant far more often, because the ability to digest milk into adulthood provided a nutritional advantage wherever dairying became a staple of the food supply. This is the kind of finding that also shows up in the interactive world maps of genotype and phenotype frequency built by the Granada research group, which chart just how unevenly this one variant is spread across the globe (NIH/PMC).

Lactose intolerance versus a milk allergy: not the same thing

Lactose intolerance and a milk allergy get confused constantly, but they involve entirely different body systems. Lactose intolerance is a digestive issue: without enough active lactase enzyme, undigested lactose passes into the colon, where gut bacteria ferment it, producing gas, bloating, and cramping. A milk allergy is an immune response, where the immune system reacts to milk proteins like casein or whey, and it can trigger symptoms ranging from hives to, in severe cases, anaphylaxis. The genetic marker discussed in this piece, rs4988235 near the LCT gene, relates only to lactose digestion. It has nothing to do with whether someone's immune system reacts to milk proteins, and a DNA test for lactase persistence says nothing about allergy risk.

What a DNA test can and can't tell you

Biome's DNA panel reports on lactose digestion using the rs4988235 marker in the MCM6/LCT region, one of the better-established examples of a single variant with a clear, well-replicated mechanism. Because dairy consumption habits, population ancestry, and the underlying gene frequencies vary so much by region, results are most meaningful when read with your own ancestry and diet in mind rather than as a universal number.

A result showing the non-persistent genotype doesn't mean you have to avoid dairy entirely. Many people manage lactose comfortably in smaller amounts, in aged cheeses that contain less lactose, or alongside other foods that slow digestion. Reputable DNA testing companies typically frame this kind of result as a predisposition rather than a diagnosis, and if digestive symptoms are actually bothering you, a breath test or a supervised elimination trial with a doctor remains the more direct way to confirm what's going on. For anything related to how a specific company stores, uses, or shares your genetic data, check that company's own privacy policy rather than assuming it matches another provider's practice.

FAQ

Is lactose intolerance genetic or something you develop? It's largely genetic. The rs4988235 variant near the LCT gene, described in a 2024 study on lactase deficiency, determines whether lactase production continues into adulthood, and most humans who lack the persistence variant naturally decline in lactase output after weaning (NIH/PMC).

Can a DNA test diagnose lactose intolerance? Not by itself. Genetic testing for LCT variants correlates well with hydrogen breath test results, according to a 2023 study in Children, but genotype and self-reported symptoms don't always match, which is why a 2019 Mayo Clinic study compared genetic testing against both breath testing and self-report (NIH/PMC, NIH/PMC).

Why are some populations more lactose intolerant than others? Populations with a long history of dairy farming tend to carry the lactase persistence variant more often, while populations without that history, such as indigenous Siberian groups studied in 2024, show much higher rates of lactase deficiency (NIH/PMC).

Is lactose intolerance the same as a milk allergy? No. Lactose intolerance is a digestive enzyme deficiency involving the LCT gene, while a milk allergy is an immune reaction to milk proteins. They involve different body systems and different symptoms entirely.

What does the rs4988235 variant actually do? It sits in an intron of the neighboring MCM6 gene and acts as a regulatory switch. The T allele keeps the LCT gene turned on into adulthood, which a 2023 study describes as a strong predictor of persistently high lactase enzyme expression, while the C allele allows LCT to shut off the way it does in most mammals after weaning (NIH/PMC).