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Certain aneuploidies such as trisomy-21 (Downs syndrome), trisomy-18 (Edward syndrome), Turner syndrome (XO) etc are more common than others.

I had a vague thought that it is related to chromosome size: the chromosomes which are towards the extremes of size will be affected in an improper segregation event.

Chromosomes 13, 18, 21 and Y are among the smaller chromosomes in humans, and are commonly associated with aneuploidy (but 22 which is also small is not affected). The larger ones are unaffected (X is an anomaly).

I thought that longer ones should be more affected because more cohesin bridges have to be broken, but it seems that is not the case.

  • Is there any association of chromosome size and frequency of aneuploidy?

EDIT

  • If the difference is because of differential lethalities then does the length correlation still hold true? Smaller the chromosomes, lesser the genes and hence lesser would be the effect of extra copy on the cellular biochemical networks.
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re: lethalities, I'd say it still depends on the genes and the dosage (extra or missing copy). Not all gene and dosage combinations are equally essential/lethal. I don't think average "biochemical activity" is going to be useful. What would be interesting is if someone had measured the fitness of specific gene deletions/duplications. Also, are genes "essential" for the completion of embryonic development equally distributed among the chromosomes? Unequal distribution would obscure any length correlation. –  dd3 May 10 '13 at 0:12
    
totally agree that biochemical network perturbation depends on the genes that are perturbed.. but most parsimonious assumption would be that all "essential" genes are equally distributed.. I shall anyways check.. that would be interesting in a systems biology point of view.. –  WYSIWYG May 10 '13 at 4:20
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1 Answer

I thought it was just that the other aneuploidies were fatal. They occur as often but miscarry earlier in the term therefore you don't see them. So basically there are only three that are viable 21,18 and the sex chromosome with a number of combinations: XO, XXY, XYY and XXX.

More information here: http://www.nature.com/scitable/topicpage/chromosomal-abnormalities-aneuploidies-290

With regard to the frequency for each chromosome:

For males (i.e., sperm; source):

The mean disomy frequency is 0.15% for each of the autosomes and 0.26% for the sex chromosomes. Most chromosomes analyzed have an equal distribution of disomy with the exception of chromosomes 14, 21, 22 and the sex chromosomes, which display significantly higher disomy frequencies.

Size of the chromosome is frequently reported as a factor in the frequency.

For females (i.e., eggs; source):

The data is obviously harder to attain. One study from IVF treatments show differences in the frequency of different chromosomes at different stages in meiosis.

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hmmm.. why other aneuploidies are lethal is also an interesting question?? Is there any study which says that cytologically other aneuploidies are equally frequent ?? –  WYSIWYG May 8 '13 at 15:55
    
there is some statistics in the link that you provided.. it doesn't actually say that other aneuploidies were reported in stillbirths or spontaneous abortions –  WYSIWYG May 8 '13 at 15:58
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this is still quite a likely explanation - there is very little or no data for pregnancies that terminate before the foetus is smaller than a pea. Almost all extra chromosomes for instance come from sperms and ova formation and they may simply not compete for fertilization. –  shigeta May 8 '13 at 17:11
    
This is the answer. They all happen with similar frequency, however most are lethal. The less lethal the more common, hence trisomy 21 or Downs being fairly common; however most trisomy 21 foetuses do die before getting to the second trimester. –  AndroidPenguin May 8 '13 at 17:29
    
@AndroidPenguin.. The two papers in the answer suggest that the probabilities are not uniform.. It seems so that the aneuploidy prone chromosomes are relatively smaller.. –  WYSIWYG May 8 '13 at 18:57
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