Aniridia is a genetic condition and in most cases is caused by damage to the PAX6 gene. This gene contains the genetic code for a protein also called PAX6 which is plays an important role in the development of the eye.
Every person has two copies (alleles) of each gene including PAX6. Aniridia is known as a dominant genetic trait because a person only needs damage to one copy of PAX6 to show symptoms of aniridia.
A person may be described as having either sporadic aniridia or familial aniridia.
Sporadic aniridia refers to cases where a genetic mutation happens at random and a baby is born with aniridia when there is no family history.
Familial aniridia refers to cases where a baby is born with aniridia which has been inherited from a parent who also has aniridia. A person with aniridia has an equal chance of passing on either their healthy copy of PAX6 or their damaged copy to their child. If the other parent does not have aniridia then this means the child has a 50% chance of inheriting aniridia. If the child does not inherit aniridia there is no risk of them passing aniridia on to future generations.
Older medical literature may suggest aniridia can also be recessive (symptoms only occur when a person has two copies of a faulty gene) and can skip generations. Since the discovery of the PAX6 gene this has been proved not to be true.
The above information describes only the most common symptoms of aniridia and the most common genetic cause. There are cases of aniridia which do not show many of the classic symptoms or which show more unusal symptoms. Genetic testing can help to establish whether symptoms are due to a PAX6 mutation or not. Some people who do display classic symptoms of aniridia do not have damage to their PAX6 gene but may have damage to other nearby genes or other genes which are important for the PAX6 protein to work correctly.
Gillespie syndrome is a condition which is characterised by partial aniridia (only part of the iris is missing), cerebellar ataxia (motor and co-ordination problems) and often a learning disability. This condition can be recessively inherited and is not necessarily linked to the PAX6 gene.