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Genetics/punnet square question

Discussion in 'MCAT Study Question Q&A' started by SephirothXR, Aug 12, 2011.

  1. SephirothXR

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    Hemophilia is a sex-linked disease. A normal man marries a woman whose father had it. What are the chances that they have 3 sons with hemophilia?

    a)1/8
    b)1/4
    c)3/8
    d)1/2

    Punnet square should have a XX* vs an XY, the "*"referring to the hemophilic gene. The actual answer explanation is that since only 2 of them will be sons, the odds that one of the sons gets it is 1/2, so it's therefore 1/2 * 1/2 * 1/2 to get A. I guessed A b/c it was the smallest, but I reasoned that hte odds of getting a son with hemophilia was 1/4 since you could also get daughters. With questions like these, how do I know to assume that difference? Where the getting the son assumption is 100% (rather than 50%) and getting a son with hemophilia is one of the two sons and not one out of the possible kids?
     
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  3. inaccensa

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    The probability must be counted 3 times for each son
    .5*.5*.5 (half a probability each time for a male) .5 probability of inheriting the disease among the sons based on punett sq. I would do it this way...hmmm
     
  4. tn4596

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    this question come with a pedigree or something?
     
  5. geeyouknit

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    I think that's just an assumption they make. They say 3 sons with hemophilia, so I would just stick with "son" and not "son or daughter". I see your point though. Maybe they took out the chance of that mistake happening by not including 1/64 as a chance.
     
  6. inaccensa

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    huh?
     
  7. salim271

    salim271 Patience is tough. :/
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    1/2 = chance for boy.

    1/2 = chance for disease.

    1/2 * 1/2 * 1/2 * 1/2 * 1/2 * 1/2 = 1/64 for 3 boys with the disease and no girls.

    OP, dont overcomplicate things by reading into questions and answer choices. Most genetics problems are simple and come down to very basic math. If they dont make something clear (they dont specifically state to include the math for no girls) then dont assume you have to do it.
     

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