📚 How to solve these questions: 12 tips for all 6 Interactive Reading question types, with a worked example for each.
The phrase a gene for something has survived in public discussion long after it stopped being useful inside genetics. It suggested a 1 mapping, one gene producing one trait, which does hold for a small number of conditions and describes almost nothing else. For traits people actually care about, height, risk of common disease, aspects of temperament, the picture is thousands of variants each contributing an effect too small to matter 2. This shift has practical consequences that are still being absorbed. A polygenic score aggregates those tiny effects into a single 3, and such scores now predict certain outcomes better than family history does. They also predict poorly for people whose ancestry is unlike the populations the score was built from, which is most of the world, since the underlying studies drew overwhelmingly from European populations. There is a deeper limitation that better sampling would not remove. A score predicts a distribution across a population rather than an outcome for a person, and the two are routinely confused. Being in the top few percent of genetic risk for a condition may mean a lifetime probability that remains 4 in absolute terms. None of this makes the scores useless. It makes them a tool whose value depends entirely on what is done with the number, and the same score can be a reason to screen someone earlier or a reason to refuse them insurance, which is a question about 5 rather than about biology.