THE GENETIC OF MENTAL RETARDATION

Mental retardation is a common problem with major implications for a nation’s health, education and community services. The causes of mental retardation have been found to have a definite etiological basis, which may be biochemical, chromosomal, Mendelian genetic disorders, or due to environmental effects such as toxins, infections, trauma or perinatal anoxia. The underlying causes remain unknown in a significant percentage of cases, but genetic causes account for 25-50 % of such cases. Chromosome aberration is a genetic abnormality which is usually a de novo or a spontaneous mutation. Recurrence risk is very low and not inherited if parent is not balanced carrier. Chromosomal disorders can be detected using light microscope, but cryptic chromosomal aberration less than 4 Mb cannot be seen. Fluorescence In Situ Hybridization (FISH) techniques using fluorescence labeled DNA sequence can be used for identifying a small abnormality in chromosomal DNA. Biochemical disorders and other Mendelian inheritance are inherited from generation to generation depending on their recessive or dominant type. Their mutation can only be detected by molecular analysis such as DNA amplification using Polymerase Chain Reaction (PCR). These disorders carry high risk of recurrence. Individuals with dysmorphic features and a family history of mental retardation without chromosomal abnormalities are possibly due to cryptic/submicroscopic chromosomal aberration as well as Mendelian disorders