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Unformatted text preview: 491 15.1 Changes in chromosome number copies of each sex chromosome, such as XXY, XYY, XXX, or XO (the O stands for absence of a chromo- some and is included to show that the single X symbol is not a typographical error). NONDISJUNCTION The cause of most aneuploidy is nondisjunction in the course of meiosis or mitosis. Dis- junction is another word for the normal segregation of homologous chromosomes or chromatids to opposite poles at meiotic or mitotic divisions. Nondisjunction is a failure of this process, in which two chromosomes or chromatids incorrectly go to one pole and none to the other. Mitotic nondisjunction during development results in aneuploid sections of the body (aneuploid sectors ). Meiotic nondisjunction is more commonly encountered. It results in aneuploid meiotic products, leading to de- scendants in which the entire organism is aneuploid. In meiotic nondisjunction, the chromosomes may fail to disjoin at either the first or the second meiotic division (Figure 15-13). Either way, n 2...
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- Spring '08