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Meiosis

Discussion in 'MCAT Study Question Q&A' started by Revilla, Dec 30, 2008.

  1. Revilla

    Revilla New Member
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    I think I'm right about this, but I want to make absolutely sure. On Kaplan's molecular genetics quiz, they have a question where the answer is:

    the further apart genes are from each other, the more likely it is they'll recombine.

    Shouldn't it be exactly the opposite?
     
  2. G1SG2

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    Nope, it turns out that the farther they are, the more likely it is that they will recombine.
     
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  3. Revilla

    Revilla New Member
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    I went back to my biology textbook last night to get a firmer understanding. So the closer they are, the more likely they are to cross over together and the further apart they are, the more likely they are to recombine? Does that sound right?
     
  4. rocuronium

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    Yes. A greater distance between two genes means a greater chance of crossing over between those genes (a higher frequency of recombination).
     
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  5. futuredoctor10

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    Yep! I think that is right.

    Genes closer together are more likely to cross over together and are said to be linked genes.
    Genes further apart are more likely to recombine.

    I am not fully understanding this... isn't genetic recombination the same thing as "crossing over"?

    I agree genes further apart are more likely to recombine... but I do not understand how genes close together are more likely to "cross over".

    UNLESS it is saying genes closer together on the same chromosome are more likely to cross over and are "linked" genes, whereas genes further apart on different chromosomes are more likely to recombine??
    - which I do not think it is:
    "When two genes are located on the same chromosome, the chance of a crossover producing recombination between the genes is directly related to the distance between the two genes."
     
    #5 futuredoctor10, Dec 31, 2008
    Last edited: Dec 31, 2008
  6. rocuronium

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    I don't think that I read the OP well enough before I responded to it. Crossing over and recombination are essentially the same thing. Recombination is what you get from crossing over.

    If you think about a random event (crossing over) occurring somewhere along the paired chromosomes, it should make sense that it is more likely for that random even to happen between two genes that have a large distance between them than two genes with very little distance between them.
     
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  7. futuredoctor10

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    OK cool this makes sense.

    This statement was in EK Biology 7th ed Lecture 2:
    "Genes closer together are more likely to cross over together and are said to be linked genes."

    Is this a typo?
     
  8. G1SG2

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    Yeah, it probably is a typo.
     
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  9. TooMuchResearch

    TooMuchResearch i'm goin' to Kathmandu...
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    No, it is not a typo. Imagine that G represents a gene of interest, and any - represents genes on a chromosome we don't care about.

    If our genes are positioned on the chromosome as shown, and two chromosomes crossover at X, then our genes near each other are likely to crossover together. In other words they are linked:

    G-G---------
    X
    -------------

    Now, if our genes our further apart and the crossover occurs at the same region of the chromosome:

    G-------G---
    X
    ------------

    Each chromosome after the crossover will contain only one of the genes of interest. In other words, the genes did not crossover together. They are not linked.

    If our genes started on two different chromosomes, they are more likely to recombine back onto the same chromosome when they are further apart on the chromosomes. In other words, the further apart two genes are on homologous chromosomes (shown below), the more likely they are to recombine.

    --G--------------

    --------------G--

    will recombine more than


    --G--------------
    -----G-----------
     
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  10. Revilla

    Revilla New Member
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    That's kind of what I got when I read the text again. It's all about the way the question is asked I guess. Thanks everyone! Your explanations helped a great deal!
     
  11. futuredoctor10

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    Your diagrams help alot, Toomuchresearch! That clarifies everything.
     
  12. grikmok

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    Everyone,

    I have a question, what is a tetrad? 'XX' or 'XXXX'?


    Thanks!
     
  13. FutureScaresMe

    FutureScaresMe UCSF '14
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    XX
    Also referred to as 'bivalent'
     
  14. grikmok

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    That's what I thought, thanks for confirming.

    I am confused then, on an EK 7th Ed. Lecture 2 question (#45) that says a germ cell in metaphase I should look like 'XXXX'. Why is that? Is it because 2N is represent as 4 chromosomes?
     

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