Genetics

How is Hunter syndrome inherited?

Hunter syndrome is a genetic disease. In Hunter syndrome, the gene that codes for the enzyme iduronate-2-sulfatase (I2S) contains a mutation (a change), which means that the enzyme is produced with errors that stop it working properly, or the enzyme is not produced at all.

In summary, the gene that causes Hunter syndrome is located on the X chromosome, so a male who inherits an affected chromosome will develop Hunter syndrome. Females have two X chromosomes, so if a female inherits an affected chromosome, in the vast majority of cases, she will not develop the disease, but will be a carrier (i.e. may pass the mutated gene on to her children). This is explained in more detail below.

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.

Learn more about the signs and symptoms of Hunter syndrome

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Hunter syndrome family tree

This family tree diagram shows how the gene responsible for Hunter syndrome is inherited and passed on.

Females can carry one Hunter syndrome gene without being affected. If one Hunter syndrome gene is passed onto a male, they will be affected by Hunter syndrome.


Mother is a carrier	Father is unaffected


50% chance a daughter will be neither a carrier nor affected	50% chance a daughter will be a carrier	50% chance a son will be unaffected	50% chance a son will have Hunter syndrome

Mother is a carrier

Father is unaffected

50% chance a daughter will be neither a carrier nor affected

50% chance a daughter will be a carrier

50% chance a son will be unaffected

50% chance a son will have Hunter syndrome


Mother is neither a carrier nor affected	Father has Hunter syndrome


All daughters will be carriers	All sons will be unaffected

Mother is neither a carrier nor affected

Father has Hunter syndrome

All daughters will be carriers

All sons will be unaffected


Mother is a carrier	Father has Hunter syndrome


50% chance a daughter will have Hunter syndrome	50% chance a daughter will be a carrier	50% chance a son will be unaffected	50% chance a son will have Hunter syndrome

Mother is a carrier

Father has Hunter syndrome

50% chance a daughter will have Hunter syndrome

50% chance a daughter will be a carrier

50% chance a son will be unaffected

50% chance a son will have Hunter syndrome


Mother is neither a carrier nor affected	Father is unaffected


All daughters will be neither a carrier nor affected	All sons will be unaffected

Mother is neither a carrier nor affected

Father is unaffected

All daughters will be neither a carrier nor affected

All sons will be unaffected

SCENARIO 1


Mother is a carrier	Father is unaffected


50% chance a daughter will be neither a carrier nor affected	50% chance a daughter will be a carrier	50% chance a son will be unaffected	50% chance a son will have Hunter syndrome

Mother is a carrier

Father is unaffected

50% chance a daughter will be neither a carrier nor affected

50% chance a daughter will be a carrier

50% chance a son will be unaffected

50% chance a son will have Hunter syndrome

SCENARIO 2

Mother is neither a carrier nor affected Father has Hunter syndrome

All daughters will be carriers All sons will be unaffected

Mother is neither a carrier nor affected

Father has Hunter syndrome

All daughters will be carriers

All sons will be unaffected

SCENARIO 3


Mother is a carrier	Father has Hunter syndrome


50% chance a daughter will have Hunter syndrome	50% chance a daughter will be a carrier	50% chance a son will be unaffected	50% chance a son will have Hunter syndrome

Mother is a carrier

Father has Hunter syndrome

50% chance a daughter will have Hunter syndrome

50% chance a daughter will be a carrier

50% chance a son will be unaffected

50% chance a son will have Hunter syndrome

SCENARIO 4

Mother is neither a carrier nor affected Father is unaffected

All daughters will be neither a carrier nor affected All sons will be unaffected

Mother is neither a carrier nor affected

Father is unaffected

All daughters will be neither a carrier nor affected

All sons will be unaffected

How is Hunter syndrome inherited?

Hunter syndrome family tree

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