Define intellectual disability and summarize its causes, including Down syndrome and fragile X.

Question

What will be an ideal response?

Answer 1

ID is a limitation in intellectual functioning and adaptive behavior that is developmental in origin. The criteria set by the DSM-5 are an IQ at or below 70, deficits in adaptive functioning, and onset during the developmental period. Causes include: 1. low- and middle-income culture; 2. childhood infections; 3. birth complications; 4. extreme malnutrition; 5. maternal alcohol abuse during prenatal development. From a genetic perspective, mild forms of ID involve common forms of inheritance, while more severe forms involve de novo gene mutations that occur in the individual.
One of the most common genetic causes is Down syndrome - the presence of an extra 21st chromosome, causing IQ as low as 40-55. Factors also include the development of amyloid plaques, less production of 2 proteins involved in neuronal survival, inadequate norepinephrine release in the hippocampus, and differences in DNA methylation compared with otherwise healthy controls.
A common genetic cause is fragile X syndrome - due to a mutation in the FMR1 gene. In this case, the mutation involves about 200 repeats of the nucleotide sequence C, G, G. This causes the expression of the gene to be turned off, and thus, no protein production. IQ levels are typically below 75.