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Androgen insensitivity syndrome (AIS) is caused by a genetic fault that means the body can't respond to testosterone properly. The faulty gene is usually passed on to a child by their mother.

Testosterone is the male sex hormone produced by the testicles. It controls the development of the usual changes expected in boys, such as penis growth and the testicles moving into the scrotum.

How babies with AIS develop

In early pregnancy, all unborn babies have identical genitals. As they grow inside the womb, they develop either male or female genitals, depending on the pair of sex chromosomes they receive from their parents and their ability to respond to the sex hormones they make.

Sex chromosomes are bundles of genes – called X or Y – that play a vital role in a baby's sexual development. Females usually have 2 X chromosomes (XX), while males usually have an X and a Y chromosome (XY).

Children with AIS have male (XY) chromosomes, but the genetic fault they inherit prevents their body responding to the testosterone they make.

This means male sexual development doesn't happen as normal. The genitals appear female or between male and female, but a womb and ovaries don't develop internally.

How AIS is passed on

In most cases, the genetic fault is passed on to a child by their mother. The faulty gene is found on the mother's X chromosome.

As the mother has 2 X chromosomes, the normal chromosome is able to make up for the faulty one, so she's a carrier of the faulty gene, but doesn't have AIS and is able to have children.

Any genetically female (XX) children the mother has will also inherit 2 X chromosomes and will be unaffected, although they too may be carriers and be able to pass the genetic fault on to any children they have.

If the mother has a genetically male (XY) child, there's a chance they could pass on the faulty X chromosome, in addition to the Y chromosome the child gets from their father.

If this happens, the Y chromosome won't be able to make up for the faulty X chromosome and the baby will develop AIS.

This means women who carry the faulty X chromosome have a:

  • 1 in 4 chance of giving birth to a girl who's unaffected, but can pass on the altered gene to her children
  • 1 in 4 chance of having a boy who's unaffected
  • 1 in 4 chance of having a girl who's unaffected and doesn't carry the altered gene
  • 1 in 4 chance of having a child with AIS

This is known as X-linked inheritance.

Read about genetic inheritance for more information.