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Sex chromosome abnormalities (SCA) describe those conditions in which there is some variation among the normal euploidic sex chromosomes in male or female individuals. These variations are most often caused by non-disjunction of the 23rd sex chromosome pair during meiosis. Found in 1 in 400 live newborns, SCA are even more common than Down syndrome. While SCA can include a variety of abnormalities of the sex chromosomes, by far the most commonly occurring SCA involve the deletion (45,X or partial X monosomy) or addition (47,XXY, 47,XYY, 47,XXX) of an X or Y chromosome. Of these conditions, only Turner syndrome, caused by the loss of all or part of an X chromosome, results in an easily identifiable physical phenotype. Subtle language, neuromotor, and learning difficulties have been identified in most forms of SCA, however. The neurodevelopmental effects of the 47XXY (Klinefelter Syndrome) and 47XXX karyotypes also have been invesitgated in light of these phenotypic features. Past studies in our laboratory have mainly focused on individuals with 45X, 47XXY, and 47XXX chromosomal profiles. Research related to 45X (Turner syndrome) is described in another section. Structural MRI studies of individuals with 47XXX and 47XXY.
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