The sometimes severe retardation known as Down’s syndrome (once called ”mongolism”) is not genetic, even though it is known to be caused by the presence of an extra (47th) chromosome in the cells of the affected child. This extra chromosome is not ”inherited,” but results from a biological accident whose mechanisms are not clearly understood. Down’s syndrome cannot be passed on from one generation to another simply because those affected are usually sterile and so cannot reproduce. It is not known why, but the risk of having a Down’s syndrome child is very much higher for women in their 40s than for younger women. The extra chromosome produces a number of characteristic physical abnormalities as well as mental retardation. Recent advances in education and health care have greatly improved the abilities and the physical health of people who have Down’s syndrome. Phenylketonuria (PKU) is a rare form of retardation, thought to depend on the inheritance of a particular recessive gene. It was first noted in 1934 that a few retarded children some of whom were siblings, excreted phenylpyruvic acid in their urine. We now know that the mental retardation of such children is in some way produced by the overaccumulation of that amino acid in their bodies.
Thus a defect in metabolism caused by inheritance of a particular gene must ultimately injure the central nervous system in such a way as to produce this form of mental retardation. Happily, a successful treatment for this genetic defect is available. The child must be fed a diet very low in phenylalanine. The dietary treatment, if begun early enough, prevents the accumulation of the responsible amino acid and the child develops with a normal IQ. PKU, it must be stressed, is a rare form of retardation. Unfortunately, there are no similarly successful treatments- and no such clearly understood causes-for most forms of retardation including Down’s syndrome.
There is an important lesson to be learned from these cases. PKU is a known inherited defect. yet it can be successfully treated. Down’s syndrone is known not to be inherited, yet it cannot be successfully treated, at least not yet. The lesson is clear: Whether a trait or set of traits is inherited or not tells us nothing about whether that trait can be affected by environmental intervention. If we understand the causes of any particular defer, then we have a good chance of learning how to treat that defect whether it is inherited or not.
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