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NBME 21 Answers

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

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 +27 
submitted by βˆ—tinydoc(259),
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Nhoacpretiu naiP rftea otresk si lrnetac Psot kreots niap oemynrdS

uascde by nctelrrlaatoa cmalitha ilssneo

P.g 045 9F1A


 +18 
submitted by βˆ—drmantistoboggan4(18),
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tI iasd ti asw talfa ot smael ni ,eortu dna hte oqstieun sdaek abuto eilv nrbo iso.rgnfpf Sceni hte elams ntr’ae nebgi rnbo in het srfit pe,alc I aids %05 lfemesa dna 0% sae.ml

hungrybox  fuck i got baited +34  
jcrll  "live-born offspring" ← baited +27  
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. +3  
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. +4  
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 βˆ—nwinkelmann(334),
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egPreeid = XLD n(to all ognsetaeinr affdeetc = nXliekd,- tcefafs laesm dan lefmaes liilmarsy = )inm.tdnao

efdceAft frehsat = 0%01 osssirtnmani to s,heurtgad 0% ntsamsniiors ot son.s
fAefdcte ershomt = %05 to tuds,agrhe 50% snrnaosisimt to Bn. ososth sparetn effetadc = 10%0 isarimtsnnso ot sdrgthaeu (ude ot hesr'taf X c,omreh)oosm 0%5 tassoinnisrm to noss d(eu to somthr'e X .Borho)ocmhemso t nsepatr deceffta aceh tsiagntinrtm hotb to hraeugtd oozusmhy(og d)ghatreu = 05% adn more reesv.e

If iictnoond is ylfmrnoiu latfa to maels ni or,uet nhet the 50% aedcftef edsab on tniissnrmoa a(s o)aveb liwl ied in ur,ote dan teh 05% ton tedfcefa illw hvae levi rhbt.is hTis sm,ena krsi fo eaelmf ebing deactffe = 05% dna rski of lei-bnrov lames geinb teefdcaf = .%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(16),
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ptdt/./iinimnthnkitaankte//nrwigcp.nwgiI:ioeieoeps_i




 +0 
submitted by tsp(0),
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Is hte nmwoa ni eht ftirs eogntnriae 12() ton fefaetcd ceesbua of eoniepcmtl ee)te1pna?(rcn2 sah ot heva the trita rof eth sadeise ot pldeove in hte tuefur gann.roeite

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



 -1 
submitted by βˆ—an_improved_me(75),
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neotsH eisq:tnou hWy sode it aerttm hatw ntaptre of ahtiencenri ti ahs? oFr lal ew nwo,k tis l'm/lcriatfncuaotait- eb eedtmr.ndie

I feel like teh sseaeti way ot eanswr it s:i naokildceggnw ti is ifulmyro"n ftala to aslem "r-oeutin = %0 chaecn fro l;emsa and ilegt stju cognutni eth oirtacnf of lmeseaf ni nrgetenaio III that eahv teh essdaie = 4/6 05~.%




 -3 
submitted by βˆ—divakhan(19),
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iTsh si tno DX,L ist XRL diseeas (AF 0220 agpe )95

,erHe hte teomrh si hsyozooumg fro teh tontuiam eehtofrer hse ipsaldys teh sedi.esa

eW see hatt,

  1. tI sipks riengsoetna
  2. aMlse rea emor eyesvrle decftafe ei(d ni ro)ute
  3. measlFe rae ffcetdae (lony enwh hzumso)yogo

Hree we e,se thta hmtoer acn spas cetefdeiv X roochmemos ot 50% mfaslee pam;& roeth 5%0 liwl eb cerrsair aetl(hyh X esmchooomr mrfo fh,rate cvetfeeid morf )Feoormhtr ,malse eMhrot eassps veticedef X oohmomresc so teyh dei (noyl 1 X )moemoorhsc

Fro the smela who ear ignvil II(I 16,1 wosnh in eeig),edpr htye hvea eneb luykc to vveirsu due ot ismcoi!asm );

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  



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