A unit of instruction
If parents pass on information, where is that information written, and in what units? The basic unit is the gene. A gene is a stretch of DNA — a molecule that carries instructions in a chemical code — that holds the recipe for one part of an organism. Most often a gene carries the instructions to build one particular protein, and proteins do an enormous amount of the work of a living body: they form structures, speed up reactions, carry oxygen, and much more.
Where genes live
Genes are not scattered loose inside a cell. They are strung along long DNA molecules that are packaged into structures called chromosomes. You can picture a chromosome as a very long thread on which many genes sit in a fixed order, like beads on a string. The complete set of all an organism's DNA — every chromosome, every gene — is its genome.
Two ideas are worth holding lightly for now and meeting properly later. First, a single gene usually does not act alone — many features come from many genes working together, plus the environment. Second, genes can be active in one place or time and quiet in another, which is how a single genome can build a brain cell and a skin cell from the same instruction book. We will return to both.
The complete collection of genes an individual carries is part of what we call its genotype — a word we will unpack carefully in the next guide, because the difference between the instructions you carry and the features you show is one of the most useful ideas in all of genetics.