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Chromosomal abnormalities
- Genetics of mental retardation

Chromosomal abnormalities account for 35-40% of SMR and 10% of MMR.[20],[21] Chromosomal aneusomies (loss or excess of chromosomal material) cause a gene dosage difference for a large number of genes and the phenotypic effect is pleiotropic; therefore, they always cause syndromes of multiple congenital anomalies and mental retardation.[22] The commonest autosomal abnormalities are trisomies, particularly involving chromosomes 13, 18 and 21 and these are all associated with increased maternal age. The most common aneusomy in live newborns is trisomy 21 (Down syndrome), which is invariably associated with mental retardation. Almost all chromosomal aneuploidies, which involve an alteration in the amount of chromosome material, are associated with mental retardation. It is not certain whether this is due to dosage effects of specific genes within the duplicated or deleted segments, or to a more general effect of aneuploidy per se. It is certainly true that individuals with chromosomal aneuploidy share some nonspecific features in common such as poor growth, microcephaly, epicanthic folds and unusual palmar creases, in addition to features more specific to the chromosomes involved.

Some chromosome abnormalities occur in the mosaic form (where some cells show the normal 46 chromosomes and others have an extra chromosome) and for disorders, which are usually seen in the full form, mosaicism will confer a milder phenotype. However, there are some conditions, which are lethal in the full form and are therefore found only in the mosaic form in surviving individuals. The common chromosomal abnormalities associated with mental retardation are summarized in [Table - 3].

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