Well, that's complicated. There's a gene on the x chromosome called the FMR1 gene. When the gene mutates, or malfunctions, the gene shuts down (methylates) and therefore cannot produce a protein that the brain needs for overall development. This is called fragile x syndrome and includes a long list of potential issues, including but not limited to developmental delay, autism, ADHD, and speech disorders.
Carriers of the syndrome have a different set of symptoms to manage, or sometimes have no symptoms at all. The FMR1 gene has not completely methylated, or shut off, yet. Older carriers can have a syndrome called fragile x-associated tremor/ataxia syndrome (FXTAS), which leads to memory loss, balance and cordination problems, and tremors, and female carriers are prone to fragile x- associated primary ovarian insuffiency.
Honestly, though, the National Fragile X Foundation website says it best (this is directly from the site linked):
Fragile X includes:
Fragile X is a family of genetic conditions, which can impact individuals and families in various ways. These genetic conditions are related in that they are all caused by gene changes in the same gene, called the FMR1 gene.
fragile X syndrome (FXS), the most common cause of inherited mental impairment. This impairment can range from learning disabilities to more severe cognitive or intellectual disabilities. (Sometimes referred to as mental retardation.) FXS is the most common known cause of autism or "autistic-like" behaviors. Symptoms also can include characteristic physical and behavioral features and delays in speech and language development.
fragile X-associated tremor/ataxia syndrome (FXTAS), a condition which affects balance, tremor and memory in some older male gene carriers.
fragile X-associated primary ovarian insufficiency (FXPOI), a problem with ovarian function which can lead to infertility and early menopause in some female gene carriers.
Some gene carriers do not exhibit any of these features. To learn more about carriers click here.
Fragile X can be passed on in a family by individuals who have no apparent signs of this genetic condition. In some families a number of family members appear to be affected, whereas in other families a newly diagnosed individual may be the first family member to exhibit symptoms.