A gene is a length of DNA forming part of a chromosome. In humans there are about 20,000-25,000 different genes arranged among the 23 pairs of chromosomes.
Genes contain the code to make proteins, which are involved in the development and functioning of body organs and systems. Proteins are essential to life and have many functions in the body, for example, as enzymes, regulators and structural molecules.
Genes are also the units of heredity by which characteristics, such as eye colour and blood group, are passed from one generation to the next.