1. Fragile X Syndrome

The most common chromosonal disorder next to Down's Syndrome, affecting 1 in 1,200 people. As well as distinctive facial features, and other physical abnormalities such as heart murmurs, it causes intellectual impairment in approximately 80% of affected boys and 30% of affected girls. Girls tend to be less severely affected but most will have autistic symptoms. Many affected boys will be hyperactive.

Many people who carry the gene are unaffected or only mildy affected, so testing for fragile-X is often routine on discovery of an ASD.

2. Down Syndrome

Perhaps one of the best known, and most common, chromosome disorders, Down's Syndrome causes intellectual impairment, distinctive facial features, (often) short, stocky stature and can cause physical problems such as heart murmurs and arthritis.

However, it has been estimated that at least 10% also have an ASD, which frequently goes undiagnosed.

3. Landau Kleffner Syndrome

Landau-Kleffner syndrome (LKS), also called acquired epileptiform aphasia, is a rare childhood neurological disorder characterized by the sudden or gradual development of aphasia (loss of language) and an abnormal electro-encephalogram (EEG). LKS affects the parts of the brain that control speech and comprehension. The disorder usually occurs in children between the ages of 3 and 7. Typically, these children develop normally and then, for no apparent reason, they lose the ability to understand others and to speak. While many of the affected individuals have seizures, some do not. The disorder is difficult to diagnose and may be misdiagnosed as autism, pervasive developmental disorder, hearing impairment, learning disability, auditory/verbal processing disorder, attention deficit disorder, mental retardation, childhood schizophrenia, or emotional/behavioral problems.

4. Angelman Syndrome

Angelman Syndrome is a genetic disorder which causes severe developmental delay, speech impairment, movement or balance disorder, and unusual behaviour (frequent laughter/smiling; apparent happy demeanor; easily excitable personality, often with hand flapping movements; hyperactivity; short attention span). A number of other physical, neurological, and behavioural characteristics are frequently seen. Angelman Syndrome shares some characteristics with autism and people with the syndrome may also be diagnosed autistic.