Summary: Mental Retardation is present in 2 to 3% of all children. It may be caused by a variety of causes, that may include genetic defects, intra-partum / peri-partum / post-partum complications leading to damage to infantile brain, or neonatal complications. Among genetic causes, the most well-known include Down Syndrome and Fragile-X Syndrome. Contributed November 5, 2005

Contributed By:
Deepika Singh 4
Prof. Thelma BK 1
Prof. Meena Gupta 2
Dr. Madhulika Kabra 3

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In about 25% male children with mental retardation (MR), the causes are mostly due to defects on the X chromosome. Some X-linked disorders include: Fragile X syndrome, Rett's syndrome, Coffin-Lowry syndrome, Renpenning syndrome etc. Fragile X syndrome is the most common cause of dominantly inherited mental retardation (MR). It is caused by an expansion of a small part of the FMR1 gene located on the X chromosome. Parents should look out for the following symptoms for fragile X syndrome:

Symptoms to look out for

Genetic Defect

The FMR1 gene has a 3 base (cgg) repeat region which varies from less than 50 in normal individuals to 50-200 in pre-mutation or carrier individuals and more than 200 in affected individuals (full-mutation). The syndrome is mostly transmitted through a carrier (pre-mutation) mother. Because females have two X chromosomes, and the carriers have less repeat numbers, the carrier females are normal. Thus, in Fragile X Syndrome, the mother of the child is a carrier but a normal female. Three situations may arise in such families, and this is depicted in figure 1 (N.B. The red X is the abnormal gene):


  1. The mother could transmit her normal X chromosome and so the child would be normal. OR
  2. The mother could transmit the pre-mutation X to the progeny, and the offspring would thus be carriers, like the mother. OR
  3. The pre-mutation X in the mother may change (expand) to full-mutation while being passed on to the progeny leading to mental retardation in the child.

Preventive Strategies

Catering to children with intellectual disabilities could be extremely difficult for parents and they need a lot of help. Parents in the reproductive age group and planning the family must be aware of the following facts.

DNA diagnosis

DNA diagnosis for mentally retarded individuals as well as Prenatal Diagnosis is available for fragile X syndrome is carried out free of cost as a part of Department of Biotechnology, Government of India funded project at the addresses given below. Tests for other X-linked mental retardation conditions are also available in the laboratory. Research for developing additional tests is being continuously pursued in the laboratory.

Interested parents/ caretakers/ teachers may contact us at UDAAN for guidance or proceed directly to any of the following:

  1. Department of Genetics
    University of Delhi South Campus
    Benito Juarez Road
    New Delhi 110021
    Phone: +91-11-24678201; +91-11-26111955 (ext 173)


  1. Director
    Institute of Human Behavioural and Allied Sciences
    GT Road; Dilshad Garden
    PO box 9520
    Jhilmil, Delhi 110095,
    Phone: +91-11-22112136


  1. Assistant Professor
    Pediatrics Department
    All India Institute for Medical Sciences
    Ansari Nagar; New Delhi 110029,
    Phone: +91-11-26594585; +91-11-26864851 ext. 4585 or 3264


  1. Department of Genetics
    University of Delhi South Campus
    Benito Juarez Road; New Delhi 110021

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