CoachA gene is a segment of a DNA molecule — typically some thousands of base pairs — whose sequence carries the instructions for making one protein (or, sometimes, one working molecule of RNA). It occupies a fixed position on a given chromosome. The versions of a gene that differ by a few bases are its alleles: same position, same function, slightly different sequence, hence often a slightly different protein.
Examples
Example 3.10 (Numbers)
The human genome — one complete set of chromosomes — carries base pairs and about genes; genes occupy only a few per cent of the sequence. A cell of E. coli carries one circular DNA molecule of base pairs and about 4300 genes. Two unrelated humans differ at roughly one base in a thousand: some three million positions, most of them outside genes and of no consequence, a few of them alleles that change a trait.
Example 3.13 (Uses of transgenesis)
Human insulin and growth hormone from bacteria; a gene for resistance to an insect pest inserted into maize or cotton; rice carrying genes for making a precursor of vitamin A; the GFP gene attached to another gene so that the cells where that gene is active light up under the microscope — the last use, a research tool, is the commonest of all. Each use raises its own questions of safety and of consequences for the environment; each rests on the same fact, the universality of the genetic molecule.