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NBME 21 Answers

nbme21/Block 1/Question#47

A 26-year-old woman (III-2) comes to the physician ...

50% in females but near 0 in males

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It said it was fatal to males in utero, and the question asked about live born offspring. Since the males aren’t being born in the first place, I said 50% females and 0% males.

hungrybox  fuck i got baited +8  
jcrll  "live-born offspring" ← baited +4  
sympathetikey  Same :/ +  
arkmoses  smh +  
niboonsh  why is it 50% females tho? +1  
imgdoc  felt like an idiot after i figured out why i got this wrong. +1  
temmy  oh shit! +  

 +1  upvote downvote
submitted by nwinkelmann(109),

Pedigree = XLD (not all generations affected = X-linked, affects males and females similarly = dominant).

Affected fathers = 100% transmission to daughters, 0% transmission to sons.
Affected mothers = 50% to daughters, 50% transmission to sons. Both parents affected = 100% transmission to daughters (due to father's X chromosome), 50% transmission to sons (due to mother's X chromosome). Both parents affected each transmitting both to daughter (homozygous daughter) = 50% and more severe.

If condition is uniformly fatal to males in utero, then the 50% affected based on transmision (as above) will die in utero, and the 50% not affected will have live births. This means, risk of female being affected = 50% and risk of live-born males being affected = 0%.

 +0  upvote downvote
submitted by tsp(0),

Is the woman in the first generation 1(2) not affected because of incomplete penetrance?1(2) has to have the trait for the disease to develop in the future generation.