Genes are the biological information passed on (inherited) from your parents. They affect the way your body grows, works, and looks. Every single cell of your body is a tiny building block and contains all the information you inherited. The information is contained in 46 chromosomes (23 pairs) within the core (nucleus) of each cell. Chromosomes are like containers or filing cabinets filled with all the genetic information your body needs to work properly. Chromosomes can actually be seen through a strong microscope.

Genes are the actual 'files' contained in these chromosomal filing cabinets. It is estimated that each human cell contains around 30,000 genes. Different genes have different functions. Some genes clearly determine different things about us: for example, eye colour seems to be linked to a particular gene.

However, other things about us are the outcome of several genes and our environment interacting together. For example, a person's weight and height are linked to the genes they have inherited (someone with tall parents is likely to be tall) and also to diet, exercise, childhood illnesses and so on. It seems that cancer, even if there is a strong family history, is almost always the outcome of an interaction between genes and the environment.

The actual 'words' inside our genetic 'files' are written in a 'chemical language' or 'code' that consists of four chemicals (bases), which are abbreviated to four letters: A, T, C and G. These four letters, repeated over and over again in different combinations in our cells, contain all the information our body needs to function. This coded information is also called the DNA (this is short for deoxyribonucleic acid).

A genetic or DNA test looks at the order in which the chemical 'letters' of the genetic code are found within a gene. This is a bit like trying to read a big file or book – and checking it for any spelling mistakes. For example BRCA1, the first gene identified as playing a role in hereditary breast cancer (and therefore called (BReast CAncer one), consists of around 100,000 'letters'.

Each cell carries out a specific function for the body and it uses the information contained in its DNA to know what to do. So your genes work a bit like an instruction manual. Therefore, if there is a spelling mistake contained in any of the 'files' stored in your body cells, it can make things go wrong with the cell. A cell could die or could grow into tissue that is not quite right (like a cyst) or it could grow and multiply out of control and develop into a cancer.

A genetic test tries to find these spelling mistakes or genetic changes (mutations). A genetic mutation is not in itself a cancer, but it might mean that you are more likely to develop cancerous cells than other people in the population.