Biology

A woman who is a carrier for hemophilia marries a man who does not have hemophilia. What are the chances of them having a son or daughter with hemophilia? Show your work.

I need help with this short-answer/essay/written problem. I think the answer to this problem is 50% due to the fact that a child inherits EXACTLY 50% of the chromosomes from each of both parents, but I'm not 100% sure. Many thanks to whomever lends me a helping hand with this problem!

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  1. If only the mother is a carrier, then none of the children will show the trait, but all offspring would have a 50% chance of being a carrier.

    Hh x HH

    1/2 HH

    1/2 Hh

    If the father is a carrier, than there is a 50% chance of the children being a carrier and a 25% chance of the children expressing the trait.

    Hh x Hh

    1/4 HH

    1/2 Hh

    1/2 hh

    The question doesn’t say whether or not the father is a carrier — it just says that he doesn’t express the trait.

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  2. "a man who does not have hemophilia." it says very clearly that he is not a carrier.

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  3. Oh wait never mind, I didn't catch the difference between being a carrier and actually expressing the trait/ not expressing it.

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  4. Thank you so much for your response, Anonymous! I really appreciate it!

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  5. If only the mother is a carrier, then none of the children will show the trait, but all offspring would have a 50% chance of being a carrier.

    Hh x HH

    1/2 HH

    1/2 Hh

    If the father is a carrier, than there is a 50% chance of the children being a carrier and a 25% chance of the children expressing the trait.

    Hh x Hh

    1/4 HH

    1/2 Hh

    1/4 hh <== typo correction

    The question doesn’t say whether or not the father is a carrier — it just says that he doesn’t express the trait.

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  6. To answre this question, we need to know a few facts about hemophilia.

    It is an X-linked recessive defect, which means that it is present only on the x-chromosome, and in females, which has 2 x-chromosomes, will require both copies to be defective to have the disorder. If only one x-chromosome is affected, she will be a carrier.

    On the other hand, males have only one x-chromosome (the other one is a y-chromosome). Therefore males will either exhibit the disorder, or be just normal. Males cannot be carriers.

    For the given problem, the mother is a carrier, and the father does not have the disorder.

    Using
    X=chromosome with disorder, and
    x=normal,
    we have the following Punnett square
    x y
    X Xx Xy

    x xx xy

    This means that there will be four outcomes, each with probability 25%:

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  7. continued:
    (where Xx is the carrier mother, xy is the normal father)
    Outcomes (each 25%)
    Xx = daughter is a carrier
    xx = daughter is free from hemophilia
    Xy = son had hemophelia
    xy = son is free from hemophelia

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  8. Yeah, I thought about answering it that way, but the author of the question did not state that it was a sex linked trait. Usually, it is stated with these types of questions.

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  9. I guess it is part of the exercise to test knowledge of the well-known sex-linked traits.

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  10. Probably so, but the question is a simple punett square question, and isn't something that is generally discussed in a general genetics course, but it has been a long time for me. It's usually discussed in a Human Genetics course, and the chromosome number or type of chromosome is given. But the question does say male or female offspring, so I suppose. I should have just went with my first thought and included punett square results for sex-linked traits, but didn't feel like typing that much work.

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  11. Regarding : "but didn't feel like typing that much work."
    I consider a proper answer related to Punnett squares should indicate the genotypes and interpret them in terms of phenotypes, whether asked for explicitly or not. Again, I consider that as a part of the exercise.

    For future reference, whenever a specific disorder is named, you would be expected to either know (as in a test) or look up and learn the characteristics of the disorder/disease. This way, we accumulate knowledge as we go.

    Wish you great success in your course!

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