the gene as the unit of heredity

Long before anyone had seen DNA, breeders and biologists noticed that traits pass from parent to offspring in discrete packets: a pea is either round or wrinkled, not a smooth average. Gregor Mendel called the hidden factors behind these packets the units of inheritance, and the word gene was coined for them in 1909, decades before their chemical identity was known. A gene is, at heart, the basic unit by which heredity is parcelled out from one generation to the next.

Molecular biology then gave the gene a physical body: a specific stretch of DNA. In the classic picture a gene is a segment whose sequence spells out the recipe for one product, usually a protein, together with the nearby DNA signals that say when and where to read it. Reality is richer than that tidy picture. A single gene in a complex organism is often split into coding pieces (exons) interrupted by non-coding pieces (introns), and through alternative splicing one gene can specify several different proteins. So the old slogan 'one gene, one protein' is a useful first approximation rather than a strict law.

Why hold on to the word at all? Because the gene remains the natural unit for talking about inheritance, variation, and disease. When we say a trait is heritable, that a mutation causes a condition, or that two species share a gene, we are using the gene as the bookkeeping unit that connects the abstract patterns of heredity to concrete sequences of DNA. It is the hinge between genetics, which counts traits, and molecular biology, which reads molecules.