AAMC 11 #105 Chromosome Question

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

  1. tshank

    2+ Year Member

    Jun 3, 2014
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    "Assume that a certain species with sex chromosomes R and S exist such that RR individuals develop as males and RS individuals develop a females. Which of the following mechanisms would most likely to compensate for the potential imbalance of sex chromosome gene products between males and females of this species?"

    A - Inactivation of one R chromosome in males
    B - Doubling transcription from the S chromosome in females
    C - Inactivation of the R chromosomes in females
    D - Doubling the transcription from the R chromosomes in males

    The answer is A. I understand AAMC's explanation - eliminating an R would make each individual only have one R - but, I don't understand why the others are wrong.
    Would A create 2RR and 2RS gene products? And that is the idea?
    If so, why doesn't C work? C would create RS and RS products also - which would be balanced.
    I think I'm just generally confused on this question, can someone translate it for me and show me how to think about this problem? Thanks.
  2. mehc012

    mehc012 Big Damn Hero
    5+ Year Member

    Jul 9, 2012
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    Medical Student
    I think the easiest way to approach it is to see the parallels with the XX/XY system we are familiar with.
    In humans, females have 2 X chromosomes, but one of these is inactivated in each cell - this is what gives us Barr bodies and phenomena such as calico cats.

    Knowing the above, the parallel to the theoretical RR/RS system is clear and choice A most closely matches.
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    tshank likes this.

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