MadSci Network: Genetics

Re: Do people with Downs Syndrome have different severities of mental retardati

Date: Wed Apr 4 06:49:40 2001
Posted By: Joshua Chai, Medical student, Medical Sciences, University of Cambridge
Area of science: Genetics
ID: 986063372.Ge

Down’s Syndrome is the most studied example of trisomy abnormality. 
Trisomy means that instead of having 2 (i.e. a pair) of homologous 
chromosomes, patients with trisomy have 3 (i.e. with an extra chromosome). 
Patients with Down’s Syndrome have 3 chromosome 21, hence the medical name 
Trisomy 21. The reason why Trisomy 21 is the most studied example of 
trisomy abnormality probably correlates to the fact that most of the other 
trisomy abnormalities are very severe and the frequency of live-born is 
relatively low, whereas trisomy 21 has a frequency of live-born, on 
average, of 1 in 600 (maybe implicating that chromosome 21 encodes for 
some relatively “non-vital” information?)

There are 2 common causes of Trisomy 21:-
1.	About 95% of it is caused by meiotic non-disjunction in which the 
pair of chromosome 21 fails to segregate at the FIRST meiotic division. 
Hence half the gametes have 2 copies of this chromosome, the other half 
have none. Notice that the latter are probably lethal, while the former, 
when fertilised by a normal gamete, will produce a trisomy zygote. It 
appears that such gametes are produced by the mother for the incidence of 
affected births is dependent on maternal age.
2.	The remaining 5 % of it is caused by translocation of chromosome 
21 to one of the other autosomes (commonly chromosome 14). Meiosis in this 
chromosome arrangement gives 1/3 of the gametes carrying the translocation 
together with chromosome 21, and fusion of such gamete with a normal 
gamete can generate a trisomy.

As for your question, it is sure that people with Down’s do NOT all have 
the same level of mental retardation (in fact very few clinical conditions 
produce the same level of adverse effect). I have seen patients with 
Down’s who appear almost as intelligent as normal being compared with some 
cases having severe mental retardation, although most affected individuals 
do share certain clinical features, such as a characteristic flat facial 
appearance with slanting eyes, broad hands with short fingers and a single 
palmer crease across the hand. The reason for the wide range of severity 
of mental ability as seen in Down’s patients can probably be explained by 
the position on the chromosome breakage and recombination. As I have 
mentioned above, chromosome translocation is a cause of trisomy 21 and 
that as the position of breakage on chromosome 21 differs from case to 
case, and hence the length of chromosome 21 being “added” onto, say, 
chromosome 14 is different. So it is not difficult to speculate that this 
difference in breakage and recombination in translocation process may be a 
cause for the difference in severity of mental retardation for affected 

I hope I have answered your question:-)

Joshua Chai
Medical Student
University of Cambridge
United Kingdom

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