Most human traits do not come in tidy either-or forms. Height, blood pressure and skin color vary smoothly across a population. So do the risks of most common diseases, such as type 2 diabetes and heart disease. These are complex traits, shaped by many genes and many environmental factors at once, and they need different tools from Mendel's.

Galton and the biometricians

Darwin's cousin Francis Galton studied continuous traits with statistics in the 1870s and 1880s. Comparing the heights of parents and their grown children, he found that children of very tall or very short parents tended to be less extreme than their parents, a pattern now called regression toward the mean. It remains a basic idea in statistics. Galton also founded eugenics; that history shows how quickly statistical findings about populations were turned into judgments about which people deserved to have children. After 1900 a bitter dispute broke out between Mendelians, who studied discrete traits, and Galton's followers, the biometricians, who studied continuous ones. Each side thought the other's evidence undermined its view.

The dispute was resolved in 1918 by Ronald Fisher, who showed mathematically that many Mendelian genes, each with a small effect, add up to smooth continuous variation. Think of rolling dice: one die gives six equally likely results, but the total of a hundred dice clusters in a bell curve. Traits shaped by many genes are called polygenic.

What heritability means

Heritability is the most misunderstood number in genetics. It is the share of the variation in a trait within a particular population that is linked to genetic differences. It does not describe an individual: a heritability of 80 percent for height does not mean 80 percent of your height comes from your genes. It also depends on circumstances. If everyone in a society ate equally well, nearly all remaining differences in height would be genetic and heritability would rise, even though food would still matter enormously.

Above all, a highly heritable trait can still change a great deal. Height in wealthy countries is roughly 80 percent heritable, yet average heights have climbed as nutrition and health improved. Dutch men grew by roughly 15 to 20 centimeters over a century and a half, a change far too fast to be genetic. PKU, from the last level, is close to 100 percent heritable and is prevented by diet.

In 1970 the geneticist Richard Lewontin gave a famous illustration. Take a sack of genetically varied seeds and plant half in rich soil and half in poor soil. Within each tray, differences in plant height will be largely genetic. But the gap between the trays is due entirely to the soil. High heritability within groups says nothing about the causes of differences between groups. Claims about inborn differences between nations or ethnic groups often ignore this point.

Hunting the genes

Since the mid-2000s, genome-wide association studies scan the DNA of very many people for variants linked to a trait. Known as GWAS, they check hundreds of thousands of points in each genome for variants slightly more common in people with a condition. They have found that complex traits involve enormous numbers of variants. A 2022 study of more than five million people linked about 12,000 variants to height, each with a tiny effect. Adding up a person's variants gives a polygenic score, an estimate of genetic tendency. Such scores can flag people at higher risk of some diseases, but they predict poorly for individuals and carry a serious equity problem. Because most participants in genetic studies have been of European ancestry, polygenic scores work less well for people of other ancestries, especially African ancestry.

Beyond the sequence

Genes and environment do not simply add up; they interact. A variant may matter only with a certain diet or in a certain climate. Cells also mark their DNA with chemical tags, such as methyl groups, that change how active genes are. This is epigenetics: chemical marks that change gene activity without changing the DNA sequence. Epigenetic marks help explain how cells with identical DNA become different tissues, and some respond to experiences such as famine or stress. Popular claims that such marks pass reliably from grandparents to grandchildren in humans, however, go well beyond the evidence, which remains contested.

Worked example: reading a headline

A news story says: "Study finds reading ability is 70 percent heritable." A parent worries that extra tutoring is pointless. Three questions help. First, heritable in which population? A figure from children in one country with universal schooling describes differences among those children, not children without schools. Second, does 70 percent describe my child? No: heritability describes variation across a group, not the makeup of one person. Third, does it mean tutoring cannot help? No. Height is more heritable than reading and still rose with better food. A trait can be highly heritable and highly responsive to teaching at the same time. The headline is a fact about a population under certain conditions, not a verdict on any child.