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How to tell on Southern Blot: carrier vs affected with Beta-thalassemia

Discussion in 'Step I' started by Belleza156, 08.05.12.

  1. Belleza156

    Belleza156

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    I attached the two pics of a Kaplan Question in which a Southern Blot is done to assess an unborn baby's likelihood of being affected w beta thalasemia.

    What is it about the blot that indicates the unborn is a carrier and not affected with the condition?

    Attached Files:

  2. johnnydrama

    johnnydrama I'm no Superman

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    Look at the bands. One band is shared by father and sister, the other two with the mother. Since the sister does not have the trait, this means the baby is a carrier.
  3. Belleza156

    Belleza156

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    I still don't get it. Can you explain in more detail?

    What bands would the unborn boy have to have to be affected?
    What bands does he have that make him not affected, but a carrier?

    He has 7kb band and 4kb band from mom, mom doesn't have the condition so the problem prob isnt with 7kb band.

    Dad is a carrier and mom as well so is 4kb band the one that shows carrier status? The unborn kid only has one copy and that's why he is a carrier? I just don't get it, why isn't he affected?
  4. Phloston

    Phloston SDN Lifetime Donor Lifetime Donor

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    The only thing the mom and dad share is the 4kb strip, so you know that reflects the mutant allele, especially since the daughter is not a carrier and she doesn't have it.

    Because the mom and dad are carriers, you also know that the 11 and 13kb strips must be normal ones because they don't have those in common.

    That means the 4kb strip must have been derived from the 11 or 13 kb strip because the mom and dad both also have strips that are (13 - 4) and (11 - 4), respectively.

    Therefore, when looking at the male fetus, you know the 13 and 7 kb strips have to be normal and the 4kb strip the carrier one.

    If the fetus had only the 4kb strip, you'd know he has beta-thalassaemia major, as opposed to just carrier status.

    Hope that helps,

    And good luck on your exam btw :)
  5. Belleza156

    Belleza156

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    Im 95% there, in terms of getting it. .....why does the girl only have two bands?

    If the boy had on the 4kb and it was twice as thick that would denote beta thalasemmia major?
  6. Phloston

    Phloston SDN Lifetime Donor Lifetime Donor

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    She got one from the mother and one from the father.

    The point of this question is to demonstrate that the 4kb band comes from the 13 or 11kb band, and that the mutation creates a cleavage site, which is why you see the 4kb band when the mutation is present. The only reason three bands are present in the carrier is because the mutant bad got split into two, whereas the normal band stayed intact as one.

    That understanding is also contingent on the intuition that the 13 and 11 kb bands are normal variants of the same allele.

    Correct. However, the boy would also have the 9 and 7 kb bands as well. So we'd see thick 4 kb band and thin 7 + 9 kb bands. If there is an 11 or 13 kb band present, then carrier, rather than disease, status is present.
  7. Belleza156

    Belleza156

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    Wow, this question was definitely more complicated than i thought. I looked it up in the biochem BRS, and now after combining your explanation and the BRS explanation of RFLP Mutations creating a restriction site, I get it!! This question definitely requires knowledge that a mutation in RFLP has two possible outcomes,

    1. Mutation in restriction enzyme cleavage site: normal DNA = two smaller restrction fragments, mutated DNA = one large fragment

    2. Mutation creates restriction site: normal DNA = one large fragment, mutated DNA = two smaller restriction fragments


    Not an easy question, thanks for your help!

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