Dictionary of Human Evolution and Biology

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Primary Nondisjunction

In organisms with an XX, XY system of chromosomal sex (such as humans), abnormal nondisjunction of the sex chromosomes that results in aneuploid gametes. In the homogametic sex (females) gametes are produced with two X chromosomes (24,XX), or none (22,0). In the heterogametic sex (males), primary nondisjunction during meiosis I produces gametes with no sex chromosome (22,0), or with both (24,XY); primary nondisjunction during meiosis II produces gametes with either (22,0) and (24,XX) or (22,0) and (24,XY). Primary nondisjunction is the supposed mechanism by which all sex aneuploids first arise. Once it exists in a family, it can be maintained by secondary nondisjunction.

See sex chromosome aneuploid.

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