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Next Step FL#5 B/B 54

PagingDr.F

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    Question/Explanation attached.

    The passage states it is autosomal recessive so in order to have the disease (which he does) he must be homozygote (aa)...how is this possible if the Father is AA and the mother is Aa?

    Mom (Aa) x Dad (AA) = 1/2 AA + 1/2 Aa
     

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    aldol16

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      I'm not familiar with the passage, but from looking at what you posted, it appears that the distinction here is between genotype and phenotype. A person can be a genotypic carrier for a disease but not show the phenotype of the disease. The phenotype would be the signs and symptoms resulting from this mutation. The genotype is simply the mutation. So the child here can be heterozygous for the mutation (like his mother) but not show the disease - which makes him a carrier.
       

      PagingDr.F

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        If the child were heterozygous, the Gel would be identical to the mothers. One allele (WT) has no cut site, the other allele (mutation) does - resulting in three bands.

        The correct answer choice shows that the child does not have a WT allele. This is impossible...it would imply that he inherited two mutatated alleles.

        Passage states the mutation is on chromosome 12 and is recessive (autosomal recessive). It also states he has the phenotype (sterile). To have the phenotype of an autosomal recessive disease, you have to be homozygous.
         
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        PagingDr.F

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          I wonder if the author of that question intended this to be an X-linked disease. In that case, the father contributed a Y chromosome and the mother contributed an X chromosome with the mutant allele.

          www.adaptprep.com/MCAT

          This is the only thing that would make sense IF the passage didn't state that the gene was located on chromosome 12.
           

          PagingDr.F

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            Hi, my guess is UPD (uniparental disomy), which can explain that gel. What do the other gels look like? It might be a case of eliminate the others and this is the best one. And like I said, UPD is a possible explanation.

            I think this may be a good explanation. The gene involved was responsible for interacting with the synaptonemal complex and caused errors during meiosis I. Honestly, I had never heard of UPD before and the passage did not mention it specifically. Are we supposed to know this for the mcat?

            I was between this answer and one that showed two bands for both the mother and patient but that's obviously wrong because the mother had to have the WT allele. I was just trying to figure out how the son could have inherited two copies of the recessive gene.

            Thanks for that tidbit of info
             

            Nugester

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              I think this may be a good explanation. The gene involved was responsible for interacting with the synaptonemal complex and caused errors during meiosis I. Honestly, I had never heard of UPD before and the passage did not mention it specifically. Are we supposed to know this for the mcat?

              I was between this answer and one that showed two bands for both the mother and patient but that's obviously wrong because the mother had to have the WT allele. I was just trying to figure out how the son could have inherited two copies of the recessive gene.

              Thanks for that tidbit of info

              No problem. Well, knowing outside information can help/hurt you depending on how you use it. I majored in genetics and am doing a genetics masters, so that is why I know of UPD. The key thing to focus on is meiosis I. I doubt you need to know the term UPD. More important concept is non-disjunction. Or the father isn't really the father ;).
               
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