Fragile X syndrome is a hereditary disorder affecting mostly males. Symptoms include mental retardation, distinctive facial features, and poor muscle tone. The syndrome is caused by mutations in a gene on the X chromosome. Since males have a single copy of the X chromosome, they show symptoms if gene on their X chromosome is mutated. Females have a second, usually normal, copy of the gene on their other X chromosome. Consequently, they are less likely to show symptoms of the syndrome.
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Fragile X syndrome is a hereditary syndrome that tends to affect males more severely than it does females. That's because the etiology, the cause of Fragile X syndrome, is having some extra material on the X chromosome. Females have two copies of the X chromosome in the cells of their bodies so that the abnormality that's in one copy of their X chromosome is to some degree counterbalanced by their other X chromosome. Men, however, have only one copy of the X chromosome in their cells, and therefore the extra material in the X chromosome tends to create more severe problems for men than it does for women. The typical issues that are faced by Fragile X syndrome include some degree of intellectual impairment, sometimes so severe that it's labeled as mental retardation. They can often be rather mild. There are also some particular facial features that people who are trained specifically to notice these things, such as larger ear size, can notice them, but that varies between individuals with Fragile X syndrome.
Alan E. Guttmacher, M.D.
Former Acting Director, National Human Genome Research Institute
Dr. Alan Guttmacher is the former acting director of the National Human Genome Research Institute, helping oversee the institute's efforts in advancing genome research, integrating the benefits of genome research into healthcare, and exploring the ethical, legal and social implications of human genomics. Dr. Guttmacher received his M.D. from Harvard Medical School in 1981. From 1982 to 1985, he completed an internship and residency in pediatrics at Children's Hospital Boston. In 1985, he earned a two-year National Research Service Award from the US Public Health Service as a fellow in medical genetics at Children's Hospital Boston and Harvard Medical School.