Genetic tests — what can they actually tell you and what is pure hype?

What a genetic test actually measures

When you buy a genetic test from companies like 23andMe, AncestryDNA or Danish providers, you typically get SNP genotyping — an analysis of hundreds of thousands of specific positions in your DNA where there are known genetic variations between people. A SNP is a single letter change in the DNA code: for example, some may have a C where others have a T at a given position. Each individual variant often has a very small effect, but together they can give a meaningful picture. PMID 32589994 PMID 32160082

It is important to understand that these tests do NOT sequence your entire genome. They look at preselected positions — like looking up an index instead of reading the whole book. Whole genome sequencing also exists, but typically costs 5-10 times more and requires specialized interpretation. Consumer tests are therefore a selection of the most well-researched variants. PMID 32589994 PMID 32160082

What the test can tell — and with what certainty

The most reliable results from consumer tests are about monogenic traits — traits determined primarily by a single gene. This includes things like lactose intolerance, alcohol turnover, earwax type and certain rare disease variants like HFE hemochromatosis. For these traits, the test is very precise because a single variant has a large effect. PMID 34188226 PMID 31329262

For most common diseases — type 2 diabetes, cardiovascular disease, Alzheimer's — the situation is more complex. Here, polygenic risk scores are used, which combine hundreds or thousands of variants into an overall risk profile. These scores can identify individuals at markedly increased risk, but rarely explain more than 10-20% of the total genetic risk. Lifestyle — diet, exercise, smoking, sleep — often has a greater absolute effect on risk than the genetic variants alone. PMID 34188226 PMID 31329262

Pharmacogenetics — the most practical application

For many, pharmacogenetics is the most directly applicable part of a genetic test. It's about how your genes affect your body's metabolism of medicine. For example, around 7-10% of the Danish population have a genetic variant that makes them slow metabolizers of clopidogrel — a blood-thinning drug given after blood clots. These people have a markedly worse effect of the medicine and should have an alternative treatment. PMID 36220915

Other examples include warfarin dosing, statin side effects, and codeine turnover. These are areas where genetic information can have direct clinical significance — and where several Danish hospitals already use pharmacogenetic testing before certain treatments. If you take regular medications, the pharmacogenetic portion of a consumer test may be the most actionable result. PMID 36220915

A large part of this turnover is controlled by the CYP450 enzyme system in the liver. Gene variations in e.g. CYP2D6 and CYP2C19 determine whether you are a slow, normal, or ultra-rapid metabolizer of many common medications, including antidepressants, pain relievers, and heart medications. Knowing your genetic status can help your doctor choose the right dose from the start and avoid serious side effects or a lack of therapeutic effect, which is one of the most concrete examples of personalized medicine today. PMID 36220915

What the test can't tell you

A genetic test cannot tell you how to eat, exercise or live. Most dietary and lifestyle recommendations based on genetics — nutrigenomics — have weak scientific evidence. A variant in the FTO gene can increase your statistical risk of being overweight by 20-30%, but this does not change the fact that caloric balance, protein intake and physical activity are the decisive factors. PMID 36220915

The test also cannot diagnose cancer, predict your biological age or tell you if you will get Alzheimer's. Even for APOE-e4 — the strongest genetic risk factor for Alzheimer's — there are people with two copies of the variant who never develop the disease, and people without the variant who do. Genes are probabilities, not fates. PMID 36220915

It is also crucial to distinguish between genetics (your static DNA) and epigenetics (how your genes are expressed). Your genetic code remains the same throughout your life, but lifestyle, stress, sleep and environmental influences continually change the epigenetic chemical markers on your DNA that turn the genes on and off. A genetic test gives you an overview of your biological potential and your innate weaknesses, but it cannot measure your current state of health or your biological aging rate — it requires other types of measurements such as epigenetic clocks (DNA methylation tests) or ongoing blood tests. PMID 36220915