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TBR: Genetics: Taking into account gender.....

Discussion in 'MCAT Study Question Q&A' started by justadream, Sep 6, 2014.

  1. justadream

    7+ Year Member

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    TBR Bio Book I page 113 #12

    "What is the probability that a woman whose father has hemophilia and who marries a normal man will have an affected son"

    Answer: 50%

    My answer: 25%

    Hemophilia is sex-linked recessive. I get that to have an affected son, the woman (who is a carrier) must pass her affected X chromosome to the boy. There is a 50% chance of this.

    But what about GENDER? Why don't you account for gender? If you draw out the punnet square, you will see that only one box (25% chance) is for an affected son.
     
  2. mehc012

    mehc012 Big Damn Hero
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    I believe when they specify the gender, they are trying to eliminate that step for you. It is more 'if she has a boy, what are the odds that he will be affected?"
     
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  3. OP
    OP
    justadream

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    @mehc012

    Okay. I hope it would be more clear on AAMC questions.

    I remember doing a similar question in either TPR/TBR a few months earlier where I got it wrong for not taking into account gender (and now I got it wrong for taking into account gender).
     
  4. mehc012

    mehc012 Big Damn Hero
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    Hopefully the AAMC ones will be clearer, yes, but no guarantee. The MCAT is like any other multiple choice exam - some of the questions, frankly, suck, and are hard to interpret.
     
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  5. kraskadva

    kraskadva ...
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    You're accounting for gender incorrectly.
    (i'll try to do a punnet square here...)

    Dad
    X Y​
    Mom
    X
    X-h
    XX | XY (boy, unaffected)
    (girl) |​
    --------- |----------------------------
    X-h X | X-h Y (boy, affected)​
    (girl-carrier)​
    so it's a 25% chance that she will have a child affected, but it's a 50% chance that her son would be affected.
     

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