| Cause of intellectual disability | Genetic abnormality |
|---|
| Down syndrome | Abnormality in chromosome 21 |
| Rett’s disorder (females only) | Abnormality in X chromosome (which is lethal for male fetuses) |
| Fragile X (the most common cause of inherited intellectual disability) | Repetition of a piece of genetic code on the X chromosome that becomes progressively more severe in each generation |
| Prader-Willi and Angelman syndromes | Deletion on chromosome 15 that has different consequences depending on which parent’s genes contribute the deletion |
| Phenylketonuria (PKU) | A genetically based defect in an enzyme, phenylalanine hydroxylase, that leads to a failure to convert phenylalanine to tyrosine. Unconverted phenylalanine is toxic to brain cells, leading to intellectual disability, which can be prevented if PKU is identified (through a blood test at birth) and the person adheres to a diet that restricts phenylalanine. |
| Congenital hypothyroidism | Inadequate production of thyroid hormone caused by a genetic mutation. The fetus gets thyroid hormone from the mother, but after birth, the deficiency leads to defects in the developing brain. If hypothyroidism is not detected within the first 3 months of life, the damage cannot be reversed, even with thyroid hormone replacement. |
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