FXS
FXS is the most common known inherited cause of ID, and the second most common cause of ID overall (after Down syndrome). FXS affects approximately one in 3600 males and one in 6000 females.1
FXS presents clinically with a wide range of symptoms, including global DD; difficulties with learning, speech and language; problems with coordination and sensory overload; and notably a range of emotional and behavioural difficulties.2,3
FXS follows an X-linked dominant inheritance pattern and is caused by an increase in length of the FMR1 gene on the X chromosome. The length of the FMR1 gene is divided into four categories (Figure 1). The longer the gene, the more likely the individual will have symptoms of FXS.2,3
Females who are premutation carriers of FXS can have a child affected with FXS. This is because the gene length can get longer when passed from mother to child: this lengthening only occurs in women. Therefore, it is unlikely males who are premutation carrier of FXS would have an affected child.4
Associated conditions
Females who are premutation carriers are at increased risk of FXPOI. Males (females to a lesser extent) are at increased risk of Fragile X-associated tremor/ataxia syndrome (FXTAS) later in life.2,3,4