MadSci Network: Genetics |
Nondisjunction is the trait whereby a progeny organism (fetus or baby in human terms) receives an extra chromosome or is missing a chromosome. Three copies of a chromosome is called trisomy. In most cases this is a lethal event and the fetus fails to go to term. However depending upon which chromosome is present in an extra copy, some can survive. For example Down syndrome is an extra copy of chromosome 21. XXY is the Kleinfelter syndrome, and XYY individuals also survive and do well. Trisomy 13 is Patau syndrome and Trisomy 18 is Edwards syndrome. Missing a chromosome (monosomy) is also generally lethal. Turner syndrome is the loss of sex chromosome (X only). So far as I can determine, people really don't have a mechanism nor can they ascribe an environmental role to these aneuploidies. It is fairly clear that increased age of the mother results in increased probability of Down syndrome. That probably holds true for the other aneuploid syndromes as well, however as they are much more rare I don't believe there is sufficient data to confirm this conclusion.
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