Nondisjunction - Research Article from Macmillan Science Library: Genetics

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The Mechanism of Nondisjunction

Homologous chromosomes are virtually identical chromosomes that occur in pairs, one member inherited from each parent. Humans have forty-six chromosomes, or twenty-three homologous pairs. In normal meiosis, homologous chromosomes pair up and, by attachment to the spindle fibres, become aligned at the cell equator. Prior to the first meiotic division, the members of each homologous pair migrate to opposite poles of the cell by means of the pulling action of the spindle fibers. This ensures that, upon completion of meiosis, each gamete will contain one copy of every chromosome.

Nondisjunction occurs when homologous chromosomes fail to separate, creating some cells with too many, and others with too few, chromosomes. Nondisjunction occurs when homologous chromosomes fail to separate, creating some cells with too many, and others with too few, chromosomes.

However, the segregation process is not error-free, and every so often it happens that two homologous chromosomes fail to separate (disjoin) and thus both migrate to the same pole. This gives rise to two types of...

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This section contains 1,245 words
(approx. 5 pages at 300 words per page)
Buy the Nondisjunction Encyclopedia Article
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Macmillan Science Library: Genetics
Nondisjunction from Macmillan Science Library: Genetics. Copyright © 2001-2006 by Macmillan Reference USA, an imprint of the Gale Group. All rights reserved.
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