A carrier is a person who can pass an inherited (genetic) disease on to their children but who does not have the disease. The person can also pass on carrier status.

Some diseases are caused by changes in a person's chromosomes or genes. Each person inherits 23 chromosomes from each parent and so has 23 pairs of chromosomes. One or both of the chromosomes in a pair may carry gene changes in a way that causes an inherited (genetic) disease.

The chromosomes are made up of genes. A gene is a portion of DNA that controls or helps control one trait, such as blood type. A change in a gene can cause a genetic disease. In some cases the same gene on both chromosomes in a pair must carry a change for the person to have the disease. This is known as an autosomal recessive condition. If only one of the genes in the pair is affected, the person is a carrier.

Some genetic conditions are caused by changes in the X or Y chromosome. These chromosomes determine a person's sex.