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Thread: when does independent assortment and crossover occur? plz

  1. #1 when does independent assortment and crossover occur? plz 
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    When does independent assortment and crossover occur in meiosis?

    Answers on the web are contradictory

    I think prophase 1 Zygotene phase is the only source of independent assortment

    is the only crossover phase?

    and prophase 1 Pachytene is the only place where crossover happens?

    I also believe that these are the only sources of gamete variation apart from random mutation due to cell damage - .

    Can anyone confirm?

    thank you

    Zero


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  3. #2  
    Forum Professor Zwirko's Avatar
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    Meosis I, prophase I for recombibation (I wouldn't bother with some of the sub-stages (specifically, pachytene and zygotene here) since recombination may be occurring in both; I tend to describe it as "early to mid prophase I" - far simpler to remember ).

    Meiosis I, metaphase I for random alignment of the tetrad chromatids at the metaphase plate (independent assortment).


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  4. #3  
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    Thank you again Zwirko,

    Are these the only significant sources of variation?
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    Forum Professor Zwirko's Avatar
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    I'm not sure if I'm following your line of reasoning. Do you mean types of variation - as in 1) recombination, 2) mutation, and 3) something else? Or do you mean variation being introduced outwith meiosis?

    Variation introduced by mutation (as opposed to recombination) includes a very, very broad and diverse set of mechanisms. It can occur post-meiotically, at fertilisation or during very early development. Mutations can also flow through populations by a variety of mechanisms.
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  6. #5  
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    thanks Z,
    I think you may have answered my question.

    I am trying to consider what mechanisms of variation can potentialy influence inheritence - parents > child
    I suppose the only mechanism is via the gamete so I am talking of those mechanisms that can change the gamete's behaviours.
    There seems to be the two mentioned above - crossover and independent assortment, always in actiion and making considerable changes to which allels are expressed; and then of course the possibility of random mutation of any process that can effect the efficacy/outcome of inheritence - which I presume would be rare - if these processes are stable.

    It seems to me (purely from my ignorance) that independent assortment will have hte greatest effect (globally), followed closely by crossover, and only a minor role for other random mutation.

    I am just finding my feet in all this - so I profess zilch expertise.
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