Genes are discrete units that each control a specific trait. Understanding the human genome will straightforwardly explain most human characteristics.

In biology classrooms of the 1990s, the gene occupied a clean conceptual position. You inherited genes from your parents; genes controlled traits; knowing someone's genes meant knowing something definitive about what they would become. The framework had deep roots, the one gene, one enzyme hypothesis, first articulated by George Beadle and Edward Tatum in 1941 through experiments with the bread mold Neurospora crassa, and later generalized into one gene, one polypeptide, and then simply one gene, one trait. It was a useful simplification that research biologists had already complicated substantially. In textbooks and classrooms, the complication had not arrived.

The Human Genome Project began in 1990, sponsored by the U.S. Department of Energy and the National Institutes of Health, with the stated goal of sequencing all three billion base pairs of the human genome. The project captured public imagination partly because of what people expected it to reveal: the complete instruction set for a human being. Journalists wrote about finding the gene for intelligence, the gene for depression, the gene for cancer. Politicians spoke of a future in which genetic knowledge would transform medicine by identifying the single causes of complex diseases. The gene-for construction pervaded popular discourse.

A working draft of the genome was announced in June 2000; the finished sequence was published in April 2003. The results surprised even some of the scientists involved. The human genome contained somewhere between 20,000 and 25,000 protein-coding genes, roughly the same number as a mouse, and only twice that of a nematode worm with fewer than a thousand body cells. More striking, only about two percent of the genome coded for proteins at all. The vast majority of the three billion base pairs consisted of sequences that did not produce protein directly: regions that regulated gene expression, served structural functions, represented evolutionary remnants, or were not yet understood.

The one-gene-one-trait model collapsed against these findings. Height is influenced by hundreds of genetic variants, each contributing a small fraction of the total effect. The same was true of intelligence, temperament, and susceptibility to most common diseases. Research in the 2010s identified tens of thousands of genetic variants that together explained only a portion of inherited height variation, with each individual variant contributing an effect too small to detect in any reasonably sized study. The genetics of complex traits could not be read from single genes; it emerged from the interaction of many variants across the entire genome.

Epigenetics added another layer. Chemical modifications to the DNA molecule itself, methylation of cytosine residues, modifications to the histone proteins around which DNA is coiled, alter which genes are expressed in which tissues at which times, without changing the underlying sequence. These patterns are partly heritable and partly responsive to environment, meaning that two individuals with identical DNA could develop measurably different patterns of gene expression depending on their early life conditions. The genome was not a fixed read-out but something closer to a dynamic regulatory system.

The decade after the genome's completion produced a sustained revision of what genetic information could and could not explain. The idea that sequencing a genome would reveal the person in the way a blueprint reveals a building, a metaphor that circulated widely in popular accounts of the project, turned out to misrepresent the relationship between sequence and organism. DNA provides possibilities and constraints. It does not, by itself, determine outcomes.

What the genome project gave biologists was not a simple instruction manual. It gave them a new set of questions, most of which could not even have been formulated before the sequence existed.