genome-wide association study

A genome-wide association study is a way of finding which spots in the genome are statistically linked to a trait or disease, without knowing in advance where to look. The idea is simple: scan hundreds of thousands of genetic markers across many people and ask which ones show up more often in those who have the trait.

Researchers genotype large groups — often tens or hundreds of thousands of individuals — at many variant positions, usually SNPs spread across every chromosome. They then compare variant frequencies between people with and without a trait. Variants that differ more than chance would allow are flagged as 'associated' with that trait.

GWAS have revealed that most common traits and diseases are influenced by many variants, each with a small effect, rather than a single gene. But association is not causation: a flagged marker often only sits near the real culprit, effect sizes are usually modest, and findings from one population may not transfer to another. GWAS point to neighborhoods worth investigating, not final answers.