Features Associated With MR+
+Listed separately because the study did not divide the cases into mild and moderate-severe groups for
It is clear from these tables that conditions such as seizures and cerebral palsy are strongly connected with MR. We do not know exactly why this is. It could be that some undetected trauma or infection caused both the medical condition and the MR. Another possibility is that in some cases, complications from seizures caused the MR. A third possibility is that a single genetic cause may be responsible for both the MR and the accompanying condition. Finally, the answer may be a combination of two or even all three of these possibilities.
The Recurrence Question
There is no question that MR runs in families. All studies on the subject have found that the MR risk for the brother or sister of someone with MR is much higher than that of the general population. A couple who has had one child with MR is ten times more likely to have another.
The question is whether this is the result of genetics or environment. The fact that most families share both makes this is a difficult area to study. This is made even more complicated by the fact that most studies will include parents who also have MR. It can be hard to determine if the children inherited MR from their parents or whether the cause was something in the family environment.
Some light has been shed on this subject by studies of identical twins. Identical twins, of course, share identical genetic material, but not necessarily the same environment.
Unfortunately, no studies have yet been done of MR in twins reared apart. According to our best current thinking, while some individuals with mild MR are simply at the lower end of the intelligence spectrum, most cases of MR have a specific medical or genetic cause, whether it is identified or not. This is supported not only by the demonstrated recurrence risks for MR with parents of normal intelligence but also by the presence of genetic causes that we can identify, including Trisomy 21 and Fragile X, in both the moderate-severe and mild MR groups.
We know from studies of both mild MR and moderate-severe MR that while outside factors such as disease, injury and social environment play some role in determining IQ, genetics are a significant, if not the most significant, cause of MR. Exactly how significant is a question that may be settled in the near future, as medical science continues to extend our knowledge of human genetics.
A careful history and physical examination, along with family history and laboratory testing may often reveal a cause for mental retardation. Knowledge of the cause for MR can empower parents to make informed decisions about testing other family members or testing a pregnancy for genetic causes of MR. Parents concerned about having a second child with MR should attempt to attain a diagnosis and then seek prenatal counseling with a clinical geneticist or genetic counselor.