Other genetic diseases with intellectual disability

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When it comes to genetic diseases with intellectual disability Down syndrome is often mentioned but much less so all the other diseases belonging to the same category of pathology.



  • First, almost . Therefore, Down syndrome represents one of the major causes of this illness.
  • Also, there are several hundred other genetic illnesses with intellectual disability and some concern only a very small number of patients.
  • Also, as opposed to Down syndrome discovered by Professor Jérôme Lejeune in 1958 and well known today,
  • Finally, the main reason is based on the firm belief of Prof Lejeune who affirmed:
    “I could continue for many years to discover the genetic cause of many increasingly rare diseases. But I believe that everything is linked. If I find how to cure Down syndrome, then the way will be open to cure all other genetic diseases.”


Nevertheless, the Jérôme Lejeune Institute follows many patients with these rarer diseases, especially Rett syndrome and Angelman syndrome.


Other chromosomal diseases leading to intellectual disability

The other chromosomal diseases are much rarer than Down syndrome. They can be linked to a loss of a part of a chromosome (deletion) of varying size (microdeletion).

  • Williams-Beuren syndrome
  • Smith-Magenis syndrome
  • « Cri-du-Chat » syndrome (5p deletion syndrome)
Diseases linked to defects in one gene (monogenic)

Similar to chromosomal illnesses, there are many monogenetic illnesses, each being very rare for the most part.


Genetic illnesses linked to genes with the X chromosome

Among rare illnesses, 10% of intellectual disabilities in boys are linked to genes on the X chromosome. These intellectual disabilities linked to the X chromosome make a heterogeneous group of several tens of syndromes. They have in common a more or less severe intellectual disability and a familial characteristic – several boys in the same family may be affected. The most well known, the fragile X syndrome, was first described forty years ago.
Another syndrome linked to the X chromosome is different: Rett syndrome, individualised at the same time, affects almost exclusively girls and is almost never familial.

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