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NBME 21 Answers

nbme21/Block 1/Question#47 (63.5 difficulty score)
A 26-year-old woman (III-2) comes to the ...
50% in females but near 0 in malesπŸ”,πŸ“Ί
tags: genetics pedigree 

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 +18 
submitted by drmantistoboggan4(18),
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It disa ti asw atfla to aemsl ni re,out dan teh ontiesqu saedk utoab ievl nrob no.fsirfpg Scien eht lmsae nt’rea engbi robn in het tfrsi ,clpae I isda 0%5 emalesf dan %0 al.mse

hungrybox  fuck i got baited +32  
jcrll  "live-born offspring" ← baited +26  
sympathetikey  Same :/ +  
arkmoses  smh +  
niboonsh  why is it 50% females tho? +2  
imgdoc  felt like an idiot after i figured out why i got this wrong. +2  
temmy  oh shit! +  
suckitnbme  This isn't exactly right as males can still be born as evidenced by individuals III 6,9,11. This basically an x-linked recessive disease. A carrier mother can still pass her normal X chromosome to a son (50% chance). It's just that the other 50% chance of passing an affected X chromosome results in death of the fetus in utero. Thus all males actually born will not be affected. +3  
makinallkindzofgainz  @suckitnbme, Correct, but if you're a live-born male, you 100% for sure do NOT have the disease, so the chance of a live-born male "being affected" is 0. +3  
spow  @suckitnbme it's not X-linked recessive, otherwise every single son would be affected and therefore have died in utero. It's X-linked dominant +3  
qball  Jail-baited +  
srmtn  correct @spow affected females= X linked Dominant +  



 +8 
submitted by nwinkelmann(311),
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rgPdieee = DXL on(t all nnsaieterog fefactde = lke,nXid- fctsefa samle dan efslame lmlsyiair = mdot)nia.n

fcedfAte ftshera = %100 miniarnsssot to egahrdust, %0 intsasornmsi to onss.
efActe df ersmtho = 05% ot hs,dareugt 5%0 osnsamitrisn to oBh.ssnot atnersp etcdfafe = 010% tminrosssani to uedsrthga de(u ot 'ersfath X ,mom)srcheoo 0%5 nrmioinatsss to nsso du(e ot smoer'ht X h.oBsrot om)heomc asrpetn dffectea ehac mtttaingnsir btoh ot atgurehd h(uomgsyooz )htraudge = %50 nad meor eeersv.

fI oiitnoncd is infruylmo alatf ot elmas in eout,r tehn eht 05% aeffdetc aebsd no ntnssioriam sa( )eobav iwll dei in rte,uo dna eth 5%0 nto acetffde lwil haev liev sbitrh. isTh ea,nms ikrs fo elaemf gneib ctfadefe = %05 nad rsik fo ol-veribn meals gineb cfaeedft = .0%

divakhan  I believe its not XLD, had it been there would still be 25% chance of males to be normal according to the Punnett square. (FA 2020 Page 59. +  
divakhan  ^ 50% chance of being normal.. which is NOT in the answer, it said 0% chance in males! +  
divakhan  Plus you'd see disease in EVERY generation if it was Dominant. For it to be XLD, the father I,1 would have transmitted it to his daughter. Instead the parents are carrier in Gen I. so its XLR disease (II, 3 being a homozygous XLR female) +  
plzhelp123  This is X-linked dominant (think Rett syndrome), if it were x-linked recessive, a female would need to have 2 affected x chromosomes to be symptomatic, which would mean her father would HAVE to be symptomatic as he has only one X chromosome, which is impossible as he would have perished in-utero. The reason the answer is 0 in males is because the question asks specifically about live-born males. If it had asked what is the risk of the fetus being affected then it would be 50% in males (but that 50% would not survive to birth) +2  



 +2 
submitted by ragacha(15),
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rhiw.itge_n:impsno.e/atdnninok/tpaeIi/ni/ietipcwtigk




 +0 
submitted by tsp(0),
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Is eth awonm ni the rfsti otrenngeai 2)1( ont fefacted esabceu of cptiloenme ctap1en)e2e(?rn has ot aevh het atrit fro hte aiedess to eploved in hte refutu aennre.tgio

b1ackcoffee  disease developed due to germline mosaicism (mutation in of the oocyte) +  



 -1 
submitted by an_improved_me(64),

Honest question: Why does it matter what pattern of inheritance it has? For all we know, its multi-factorial/can't be determined.

I feel like the easiest way to answer it is: acknowledging it is "uniformly fatal to males in-utero" = 0% chance for males; and legit just counting the fraction of females in generation III that have the disease = 4/6 ~50%.




 -3 
submitted by divakhan(15),
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Tshi is ont ,LXD sit RLX adesise FA( 0202 gpea ) 95

rHee, eth mertoh is osgohuoymz for het itoutamn hrerfoeet ehs ayldpssi hte ied.seas

We ees at,th

  1. It psisk oegrnitsnae
  2. laeMs are rome sveyleer cftfeaed ied( ni ru)toe
  3. seaeFml are eeftadcf n(ylo hewn yuo)hsmgozo

Heer we ,ese atth ehrtmo nac spsa edvicfete X omescomrho to %05 saeemfl apm&; othre 5%0 ilwl be racrries aylhe(ht X msohomeorc fmor tah,rfe eteefvicd orfm mh rrteFoo) ae,msl oteMrh sepssa eeedfctvi X mroecomhso so hyte die loyn( 1 X c)hosemomor

For het lesma who ear givlin II(I 6,11 nwsho ni eegdei),rp ythe avhe neeb klcuy ot rueivvs edu to ssic!immao ;)

b1ackcoffee  don't you think you are going through too many hoops (assumptions)? male survive due to mosaicism? Better chance is disease started due to germline mosaicism and it IS XLD. btw the disease is incontinentia pigmenti (not necessary to know). your second last para doesn't make sense, read again with fresh mind and perspective. +1