NBME 24 X-linked recessive (NBME Answers)

Retired NBME Answer Explanations

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Retired NBME 24 Answers

nbme24/Block 1/Question#5 (reveal difficulty score)
The 35-year-old woman indicated by the arrow ...
X-linked recessive 🔍 / 📺 / 🌳 / 📖
tags: genetics process_of_elimination

+16 upvote downvote
submitted by stars and more(16)

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X linked recessive there can never be any Male to Male transfer since the male offspring gets Y from father. the pedigree shows Male to Male transmission so this cannot be X linked recessive

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weirdmed51 ??? +

weirdmed51 The pedigree shows only affected male, hence there has been female to male transmission (females-carriers). Hence X linked recessive. +

weirdmed51 Nevermind, I got it wrong. +1

freemanpeng I beg to differ... AR is much more unlikely than XR in this case. 1. XR, you only need 2 carrier wivies 2. AR, you need(1)2 carrier wives and(2)only boys inherited that mutated gene. Poor NBME guys, knowing nothing about maths, only sticking to stupid "Male to Male transmission" law +

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+15 upvote downvote
submitted by ∗monoclonal(24)

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The point here is, they are asking us to eliminate the only answer that is not possible. I got it wrong. read well fellas

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ergogenic22 nbme logic: "what if they all marry carrier wives" And doesn't everyone with autosomal dominant get it in the 2nd generation (unless incomplete penetrance) i feel dumb +26

cavernosum totally agree. what a st*pid qs! +1

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+6 upvote downvote
submitted by obgynnycstep1(7)

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The pedigree presents us with Male-to-Male transmission. There are two modes of inheritance where Male-to-Male transmission is impossible: 1) X-linked recessive 2) Mitochondrial inheritance

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+2 upvote downvote
submitted by trazabone(16)

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My understanding is that if parents are unrelated by blood to those affected, we assume that they are not carriers (in the recessive case). Therefore, if we have a male father affected with x-linked recessive married to a non-carrier, there's no way any of his offspring would be affected.

"If one parent is not a carrier, then a child can only inherit a disease allele from the other parent. In these problems, we can assume that any individual marrying into the family is not a carrier." https://www.cs.cmu.edu/~genetics/units/instructions/instructions-CP.pdf

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linwanrun1357 If we assume that they are not carriers (in the recessive case) Then how came it can be AR？！！ +3

catscan1979 ^exactly what's said above here. I think x-linked recessive is the least likely, but not impossible. +4

furkan7 How is x linked recessive is the least likely when we need 2 carrier females for compatibility of both autosomal recessive and X linked recessive inheritence to this pedigree? I think probability of these two are the same. Am I missing something? +1

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+2 upvote downvote
submitted by ayhamai(2)

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x-linked is not possible if the woman was not a carrier autosomal recessive is not possible if the woman was not a carrier so, why we choose x-linked

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utap2001 In all pedigree questions, you check up for 3 things: 1. if the inheritance is continuous(not skip generation), if yes-> dominant; no-> recessive. 2. check male to son transmisstion, if yes-> autosominal; no-> X-linked. 3. check male to daughter transmisstion, if no-> mitochondrial. https://step1.medbullets.com/biochemistry/102036/modes-of-inheritance +2

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+1 upvote downvote
submitted by ∗therealslimshady(42)

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I thought there weren't supposed to be trick questions on these things.

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agraham416 Yeah I misread the question too. +1

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+0 upvote downvote
submitted by ∗freemanpeng(7)

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No one talks about AR? That's much more unlikely in this case than XR

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+0 upvote downvote
submitted by ∗j44n(141)

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I got thrown off because there's an arrow point to the three unaffected daughters (with an affected father) so I thought XLR because they'd be carriers. And there was no generation skipping so that made me think AR

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+0 upvote downvote
submitted by failingnbme(3)

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This is the 3rd question I didn't read properly fk fk fkityfk

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