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Question:  If a mentally retarded couple had a child, would it be mentally retarded?
Karthikk

Answer 1:
Karthikk,
 
Just a short answer.
The likelihood you ask about would vary according to what type of
mental retardation is present in each of the parents.
Some (probably most) forms of mental retardation are genetic in
nature and therefore would be inheritable.  There are of course cases
where non-mentally retarded parents produce a child which is mentally
retarded.  These cases occur through errors in either production of
the sex cells, possibly due to exposure to radiation, for example,
or in errors in cell division leading up to and through conception.
 
A good deal of research is currently underway mapping the human
chromosomes.  I would suspect in the not-so-distant future, an
exact answer could be given to your question by looking at the
chromosomes of the parents and producing at least a statistical
prediction of heritable mental retardation.
 
Ric
Thanks for using NEWTON!
 
Answer 2:
   I think this answer needs serious and careful rethinking.  I am not
qualified to fully answer the question of the heritability of retardation
myself, but I would like to point out that historically there has been
intense prejudice toward and even violence (e.g. forced sterilization) done
to people with disabilities based on ignorant if sometimes well-meaning
assumptions that these disabilities are heritable or even contagious.
   I *think* the most common causes of mental retardation at birth are,
first, "fetal alcohol syndrome," which is damage caused before birth to the
developing baby's brain by the mother drinking too much alcohol (what's
"too much" we don't know --- it may not even be enough to make the mother
drunk, the only safe bet is to use none at all while pregnant).  Second, a
certain chromosome can accidentally be tripled instead of doubled at an
early stage of the embryo's growth.  This is called "Down's syndrome."
Finally, during a very difficult birth a baby can be deprived of oxygen and
suffer brain damage.  I *think* the main cause of mental retardation after
birth is trauma (injury) to the head, or ingestion of lead from e.g. pain
chips or gasoline fumes.  I do not know if Down's syndrome is inheritable,
but I *think* not.  None of the other causes can lead to an inheritable
condition, since they affect the brain directly and not the sex cells.  My
own *guess* would be that most mental retardation is not inheritable.
   The utility of a statistical correlation between parents' genetic
background and their children's mental abilities is unclear.  You need only
remind yourself of the controversy over Murray and Herrnstein's recent book
to understand that.
Christopher Grayce