While there are great differences between these subcategories, genetics plays at least some role in both. For many individuals and families, the cause of mental retardation is not yet known. One reason for the uncertainty is that mental retardation covers a number of different problems with different causes. Looking for a Cause Many studies have attempted to find out how and why MR occurs. Unfortunately, most were done in the 1980s or earlier, before the recent advances in our ability to identify and test for certain genetic and chromosomal disorders.
Chromosomes are the materials within human cells that contain our genes.
It appears that the more severe the MR, the more likely the cause is to be genetic. The seemingly lesser role of genetics in milder retardation may have something to do with the fact that some who fall into the mild MR category may simply represent the low end of the normal range of human intelligence. In other words, their low IQ may be the result of low family IQ, not a genetic problem or other specific cause. Those in the moderate-severe category, however, are much more likely to be there because of an environmental or genetic problem that has disrupted normal development. Normal individuals have 46 chromosomes, arranged in 23 pairs, with a parent contributing one chromosome of each pair.