Noonan syndrome is caused by a faulty gene, which is usually inherited from one of the child's parents.
There's no evidence to suggest the genetic fault is caused by environmental factors, such as diet or exposure to radiation.
Faults in at least 8 different genes have been linked to Noonan syndrome. The most commonly altered genes are:
In around 1 in 5 cases, no specific genetic fault can be found.
The symptoms of Noonan syndrome are generally similar, no matter which gene is affected.
However, the faulty PTPN11 gene is commonly associated with pulmonary stenosis (a narrowed heart valve) and the faulty RAF1 gene is more often associated with cardiomyopathy (disease of the heart muscle).
Read more about the characteristics of Noonan syndrome.
In around 30-75% of cases, Noonan syndrome is inherited in what's known as an autosomal dominant pattern.
This means that only one parent has to carry a copy of one of the faulty genes to pass it on, and each child they have will have a 50% chance of being born with Noonan syndrome. The parent carrying the faulty gene will also have the condition themselves, although it may be very mild.
In the remaining cases, the condition is caused by a new genetic fault that isn't inherited from either parent.
If you have a child with Noonan syndrome and neither you or your partner have been diagnosed with it yourselves, talk to your GP about getting a referral to a genetic counsellor.
You may be able to have a genetic blood test to see if either of you carry one of the faulty genes associated with the condition.
If neither of you carry one of the faulty genes, the risk of having another child with the condition is very small (estimated to be less than 1%).
If one of you does carry a faulty gene or has been diagnosed with Noonan syndrome, there's a 50% risk of each further child you have being born with the condition.
A genetic counsellor can explain the risks and your options for having another child. These may include:
Read more about diagnosing Noonan syndrome.