The Neurobiology of Mental Retardation: Fragile X Syndrome Essay

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The Neurobiology of Mental Retardation: Fragile X Syndrome

In my previous paper, I wrote on the topic of the nature-nurture debate and the ways it related to the brain-equals-behavior dilemma. In this paper, I will continue this investigation into the link between genes and neurobiology, but I will focus in on a particular aspect of the relationship: neurological disease caused by genetic aberration. There are many well studied and well documented (thought not necessarily well understood) disorders associated with the X chromosome, and a large number of these have neurobiological roots and behavioral manifestations. One such disease is fragile X syndrome. Fragile X syndrome is the most common inherited form of learning disabilities
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Men who exhibit the behaviors associated with FXS, then have inherited the disease from their mothers. To continue with the transmission genetics pertaining to this syndrome, it is also known that there are more than twice as many female carriers of the disease than male carriers (1). Of the approximately 1 in 259 women in the population who are carriers of the fragile x mutation, some are pre-mutation carriers and others are full mutation carriers. The former class is generally unaffected by the disease. The full mutation carriers, however, exhibit a range of mild to moderate behaviors associated with learning disabilities and retardation. Thus, the disease can vary in severity, a behavioral characteristic attributable to the varying degrees of expressivity of the mutant gene responsible for the disease.

What is known of this elusive mutant gene which is thought to cause fragile X syndrome? The gene is called Fragile X Mental Retardation 1 (FMR1) and is normally transmitted stably from parent to offspring in humans as well as in divergent organisms. The mutation consists of excessive copies of the CGG triplicate (which codes for the amino acid arginine) at the 5 end of the FMR1 gene. This expansive repeat results in the inability of FMR1 to be expressed (2). Through cloning, it has been found