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Post by lotus0618 » Sat Jul 18, 2020 10:21 pm


So How would the pedigree look like if it were X-linked recessive or Y-linked?

I also found this picture online. For X-linked dominant condition, isn't this picture wrong? cause based on the Punnett square, the parents: XY and X(A) X -> the offspring: X(A)X, X(A)Y, XX, XY => 50% of daughters and 50% of sons would be affected.

AAMC FL3 BB: Question 37:
So the moment I read this question, I KNEW I had to find the palindromic sequence. But I'm not sure if I was afraid of spending so much time on it, that I couldn't find the sequence and picked one answer randomly to move on. Any suggestions on how to improve this?

AAMC FL3 C/P: question 4
What's the short cut to figure out the product?

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Post by NS_Tutor_Yuqi » Sun Jul 19, 2020 1:42 pm

CP4: I'm not sure what sort of shortcut there is, but I've attached a drawing of how to follow the extra methyl group through the different steps.

BB6: If it was X-linked recessive, then the entire third generation on the left should have been affected, since their mother would have two copies of the recessive allele and their father one.
If it was Y linked, then every male would have had it.
It seems like your confusion with the diagram is because the F1 generation only had one child? If there were more children, then a pattern like what you predicted might appear.

37: I discussed ways to become more familiar with palindromic sequences on another post, which is the best way to fix this problem. For the future, if you know the way to get to the answer, I would suggest spending a little bit of extra time solving the problem. However, if it's already been 3-4 minutes and you're nowhere close, then I would just flag it and move on.
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Post by lotus0618 » Sun Jul 19, 2020 1:53 pm

Based on the pedigree in the passage 2 AAMC FL3 BB, only the male is affected. If it was X-linked recessive, how do you know that the mother would have 2 copies of the recessive allele. Even if we know the allele is recessive, unless the passage provides us with information and some clues, how are we supposed to know if the mother is heterozygous carrier with the recessive allele or affected (with both recessive alleles)?
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Post by NS_Tutor_Nancy » Thu Jul 23, 2020 9:27 pm


For AAMC FL3 BB6 based on you follow-up question, I think you got confused as to which part of the pedigree was being referenced by Yuqi. You are correct that in the first generation, only the male is affected. So the female is possibly a carrier, but you cannot know, and she would have at least one normal X (if this were hypothetically an X-linked recessive trait). Yuqi was referring to the couple in the second generation on the left where both the male and female are affected. This would mean that neither parent has a normal X to give and therefore all their children would have the condition in generation 3. This is not the case in the pedigree, so it cannot be an X-linked recessive trait.

I hope this helps!
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