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Thread: Chromosomes

  1. #1 Chromosomes 
    Forum Freshman Jellybird's Avatar
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    I read in a textbook that Down's syndrome is caused by a person having 3 chromosome 21s. It said that sometimes when cells divide, both 21s go into the same egg cell, leaving the other with none. I was wondering, what would happen if the other egg was fertilised instead, the one that has no 21s?? Would anything happen? Anyone knows??


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  3. #2  
    Forum Ph.D.
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    I dont think this is compatible for life; there would be a spontaneous abortion early on. There wouldnt be enough genetic information in such a cell for existance.


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  4. #3  
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    It would be called monosomy 21 and i am pretty sure It would not develop past an embryo. Miscarriage!
    It is not so much that I have confidence in scientists being right, but that I have so much in nonscientists being wrong. --- Isaac Asimov
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  5. #4  
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    You guys are wrong in saying that this would definitely be a spontanious abortion. There are in fact monosomy cases!

    I can remember having covered some monosomy cases back when I was in University (Trisomy syndromes are quite common for most chromosomes, not only chromosome 21, but monosomy is much rarer).

    If I remember correctly, there were some scientifically described cases of monosomy 21 which did certainly develop quite far (if not quite full term, then pretty close to it). Some cases of apparent monosomy 21 could possibly be ascribed to chromosome 21 being incorrectly incorporated into another chromosome, which would then still leave the genetic material in the cell, but as part of another chromosome. This would probably also cause quite severe defects, but can still result in a baby being born.

    As a last interesting fact, I can just elaborate a bit on what I mentioned earlier about there being many different types of Trisomy, not only Trisomy 21. The same thing that causes Trisomy 21, namely a chromosome pair getting stuck to each other during the delayed Meiosis that takes place in a female's ovaries (the process is "paused" for several decades), which then leads to one egg cell recieving two and another not one of the specific chromosome, can also affect other chromosomes. The only reason that Trisomy 21 is so much more common than other types of Trisomy, is that chromosome 21 is one of the smallest (if not the smallest?) chromosomes, and having an extra copy of it is not quite as disruptive as other trisomies of bigger chromosomes.
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  6. #5  
    Forum Sophomore scientist-to-be's Avatar
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    Why do chances of getting a baby with down's syndrome increase after the age of 40?
    In theory, there is no difference between theory and practice. In practice, however, there is.
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  7. #6  
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    As I said before, Meiosis, the process that turns a normal cell into a sex cell that is ready to be fertilised, gets "paused" halfway in a females body for years (decades). the first half of the process takes place before birth, after which the process is paused and the second half only happens to one or maybe two egg cells at a time just before menstruation.

    During this "pause" in the process, the chromosomes themselves are very tightly packed together and not in the normal state of being largely unpacked. The last I heard was that this tightly packed state of chromosomes added to the fact that the chromosomes are also paired up tightly with their homologues partners, increases the chances of causing some chromosomes to become stuck together the longer the time is that they have to stay like that. In other words, after 30 years of being packed together they still seem to come apart quite well, but after 35 or 40 years, there is a statistically significant increase in the chance of things going wrong.
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