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Part 1. Genetics

1.2. Cytogenetics

Short Specialist Review

  1. Neil E. Lamb

Published Online: 15 APR 2005

DOI: 10.1002/047001153X.g102307

Encyclopedia of Genetics, Genomics, Proteomics and Bioinformatics

Encyclopedia of Genetics, Genomics, Proteomics and Bioinformatics

How to Cite

Lamb, N. E. 2005. Nondisjunction. Encyclopedia of Genetics, Genomics, Proteomics and Bioinformatics. 1:1.2:16.

Author Information

  1. Emory University, Atlanta, GA, USA

Publication History

  1. Published Online: 15 APR 2005


Nondisjunction is the improper segregation of chromosomes during either meiotic or mitotic cell divisions. Mitotic nondisjunction is commonly observed in cancer, while nondisjunction during meiosis results in gametes that are chromosomally unbalanced. If an unbalanced gamete participates in fertilization, the resulting embryo will be aneuploid, with either one chromosome too many (trisomy) or one chromosome too few (monosomy). Because these embryos are generally inviable, nondisjunction in humans has serious clinical consequences. Although monosomic conditions appear to result in early embryonic lethality, most trisomic conditions are compatible with at least some fetal development. DNA marker studies indicate that nondisjunction errors in the female during the first stage of meiosis predominate among nearly all trisomic conditions. In addition, most, if not all, human trisomies are affected by increasing maternal age. Very little is understood regarding the mechanisms that underlie this age effect. Recently, altered genetic recombination has been identified as the first molecular correlate of human nondisjunction. It appears likely that the risk of nondisjunction posed by altered recombination varies among the chromosomes. Other risk factors for nondisjunction have been suggested, but to date, none have been conclusively proven. As a result, an understanding of the molecular mechanisms for nondisjunction and the maternal age effect remains elusive.


  • chromosome segregation;
  • meiosis;
  • aneuploidy;
  • trisomy;
  • monosomy;
  • recombination;
  • nondisjunction;
  • maternal age effect;
  • meiotic exchange