All cells of the body contain genetic information, in the form of a chemical called DNA. DNA forms two long, twisted strands (the double helix), which are organised into structures called chromosomes. Genes are sections of DNA that serve as codes (a bit like a recipe) to make other molecules, such as enzymes. The location of a gene can be mapped to a specific position on a specific chromosome.

Humans have 23 pairs of chromosomes; one pair of which are the sex chromosomes. In females, the sex chromosomes are both X chromosomes, and in males the pair consists of one X chromosome and one Y chromosome.

The gene that codes for the enzyme I2S is called IDS and is located on the X chromosome, so Hunter syndrome is referred to as X-linked. Hunter syndrome is recessive, which means that if the individual has one healthy copy of the gene, the disease will not present.

Males have only one X chromosome. This means that if the X chromosome has the mutant IDS gene, he will develop Hunter syndrome. This is why Hunter syndrome is much more frequent in males than in females; in males only one mutant copy of the IDS gene is needed to develop Hunter syndrome.

Males only pass on X chromosomes to their daughters, so a daughter of a male with Hunter syndrome will inherit a mutated gene. Males only pass on Y chromosomes to their sons, so cannot pass on a mutated IDS gene to their sons.

A female with a mutated IDS gene on one X chromosome and a healthy gene on the other X chromosome will not have Hunter syndrome, but is known as a carrier. This means that she can pass on the mutated IDS gene to her children. She will pass on one of her X chromosomes to her child, so there is a 50% chance that the chromosome passed on will be the one with the mutant IDS gene.

In very rare cases, a female may develop Hunter syndrome. This may be because the X chromosome with the healthy copy of the IDS gene has another problem, or she may have inherited two X chromosomes that each contain a mutant gene.