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The human genome has approximately 3 billion bases that are distributed over 23 chromosome pairs, with 22 pairs of autosomes and 1 pair of sex chromosomes, X and Y. The most obvious genetic variations can be observed at the light microscope level in the karyotype. This approach visualizes metaphase chromosomes using histological procedures, allowing identification of each specific pair of chromosomes and variations in the total number of chromosomes, such as unisomies and trisomies. Several of the known variations of total chromosome number have an associated psychiatric phenotype. For example, Down syndrome is a complex neurodevelopmental disorder that results in variable levels of mental retardation, and in old age, dementia strikingly similar to Alzheimer's disease (Visootsak and Sherman 2007). Down syndrome results from trisomy 21 (i.e., inheritance of three copies of chromosome 21, due to meiotic nondysjunction during oogenesis. Turner syndrome, in which there is only a single X chromosome (i.e., an XO karyotype), is associated with nonverbal learning disabilities, particularly in arithmetic, select visuospatial skills, and processing speed (Sybert and McCauley 2004).

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