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What Is Intellectual Disability?

It is the disability which is characterized by the limitations that occur due to intellectual functioning and adaptive behaviour which affects the functioning of an individual in a community or society (Vissers et al. 2016). These disabilities can be due to various reasons and are always present and the outlook of the same can be seen before an individual becomes an adult.

Intellectual Functioning

It can also be called as intelligence and it refers to the general mental capacity of an individual like learning, reasoning, problem-solving and so on. This is usually measured in the form of intelligent quotient (Tassé et al. 2016).

Adaptive Behaviour

It is the collective capacity of a person in terms of conceptual, social, and practical skills.

  • Conceptual skills- it includes language, literacy, money, time, number of concepts and self-direction.
  • Social skills- it includes social responsibility, problem-solving capacity in society and ability to follow social rules etc.
  • Practical skills- it includes to manage and complete activities of daily living, holding employment, safety, healthcare, transportation, use of money and others (Tassé et al. 2016).


Intellectual disability can basically be classified as mild, moderate, severe and profound and this is based on the severity of the intellectual disability. As intellectual functioning is one of the key components the classification is also given based on the range of intelligent quotient (Carr and O’Reilly 2016).

Aetiology of Down’s Syndrome

Down syndrome is the intellectual disability which occurs due to random error during the division of cell and which results in the extra copy of the 21st chromosome (Asim et al. 2015). The error which causes this is called non-disjunction. During a normal division when the set of chromosomes which are two arms split into two and one of each arm goes into one cell but in this case, the error occurs during the split and there is the formation of an extra copy of a chromosome in one cell. There is no known behavioural or environmental causation of the disorder. The occurrence of the error happens during the formation of eggs in females or sperms in males (Asim et al. 2015).

In humans, there is the presence of twenty-three pairs of chromosomes and which accounts for a total of forty-six chromosomes. Each chromosome has a specific role in the proper functioning of the body, physically and psychologically (Asim et al. 2015). Down syndrome is a genetic disorder in which there is the presence of extra genetic material in that makes it more than twenty-three chromosomes and it is responsible for altered developmental changes and physical features of the individual with Down syndrome.

It is seen that in the majority of the cases the inheritance is inclined towards the mother while the paternal inheritance is attributed to only in five per cent of the cases and the least occurrence is due to the changes that occur after the fertilization of an embryo. There are three different processes of change that occurs during the division of the cells and genetic material which causes the formation of the trisomy so that there is the outlook of the disease (Asim et al. 2015). They are:

  • Complete trisomy 21- occurs during the formation of egg or sperm and there is the presence of complete one extra chromosome and when the fusion occurs there is resultant presence of a total of three chromosomes instead of regular two.
  • Mosaic trisomy 21- it accounts to less than 5% of total cases in which not all the cells of the body have three chromosomes but some of them have extra chromosomes and the outlook or the severity of the disease is dependent to an extent on this.
  • Translocation trisomy 21- in this the whole chromosome is not present as excess genetic material in the cells but it is due on a part of the chromosome is present in excess in cells and again the outlook or the severity of the disease is dependent to an extent on this.

Signs and Symptoms of The Disease

Depending on the extent of the disorder the intellectual disability also varies and can be mild, moderate and severe and it can be associated with other physiological problems like structural defects in the heart. There is also the presence of altered physical appearance that can be characteristic of the disease. They are flattened bridge of the nose, reduced size of the head, short neck, palpebral fissures, protruding tongue, presence of Brushfield's spots on the iris, less tonicity of muscles leading to excessive flexibility and short stature.

References for Intellectual Disability

Asim, A., Kumar, A., Muthuswamy, S., Jain, S. and Agarwal, S. 2015. Down syndrome: An insight of the disease. Journal of Biomedical Science22(1), p.41. https://doi.org/10.1186/s12929-015-0138-y.

Carr, A. and O’Reilly, G.A.R.Y. 2016. Diagnosis, classification and epidemiology. In The handbook of intellectual disability and clinical psychology practice (pp. 23-64). Routledge.

Tassé, M.J., Luckasson, R. and Schalock, R.L. 2016. The relation between intellectual functioning and adaptive behavior in the diagnosis of intellectual disability. Intellectual and Developmental Disabilities54(6), pp.381-390. https://doi.org/10.1352/1934-9556-54.6.381.

Vissers, L.E., Gilissen, C. and Veltman, J.A. 2016. Genetic studies in intellectual disability and related disorders. Nature Reviews Genetics17(1), pp.9-18. https://doi.org/10.1038/nrg3999

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