MadSci Network: Genetics
Query:

Re: What causes nondisjunctions to occur?

Date: Mon Aug 23 09:48:36 1999
Posted By: Michael Benedik, Faculty Biology and Biochemistry, University of Houston
Area of science: Genetics
ID: 935233106.Ge
Message:

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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