gene
/ JEEN /
If the genome is a cookbook, a gene is one recipe in it — a self-contained set of instructions for making one particular thing. Just as a recipe tells you how to make bread without telling you anything about soup, a single gene carries the instructions for one product, most often one protein, like the one that lets your blood carry oxygen or the one that gives your eyes their color.
Physically, a gene is a specific stretch of DNA — a defined sequence of bases at a particular spot on a chromosome. Its core is the coding part that spells out a protein, but a gene also includes control regions that act like an on/off switch and a volume dial, deciding when and how strongly the recipe is used. The cell reads a gene by copying it into RNA (transcription), and that RNA is usually then used to build a protein (translation). Some genes don't code for protein at all — instead their final product is a working RNA molecule.
Genes are the units of heredity: they are what gets passed from parent to offspring, and small differences in a gene's sequence (variants) help explain why individuals differ. But it's a myth that there is one gene 'for' most traits. Most characteristics — height, risk of disease, temperament — arise from many genes acting together with the environment, and one gene can influence several traits at once.
The gene called HBB carries the recipe for part of hemoglobin, the protein that ferries oxygen in red blood cells; a single-letter change in it can cause sickle-cell disease.
One gene, one recipe — and one letter can change the outcome.
Defining where a gene starts and ends is fuzzier than it sounds: thanks to overlapping reading, alternative splicing, and noncoding RNA genes, even biologists argue about exactly what counts as 'one gene.'