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NBME 18 Answers

nbme18/Block 3/Question#33 (reveal difficulty score)
4 yo boy, clumsy gait for 1 year
1/4 ๐Ÿ” / ๐Ÿ“บ / ๐ŸŒณ / ๐Ÿ“–
tags: genetics pedigree

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submitted by โˆ—sugaplum(487)

FA 2019 pg 59-61
X linked muscular dystrophy. The chance that the mom III-1 is a carrier is 1/2. XY (normal grandpa II-1) * XaX (carrier grandmom II-2) You know that III-1 is a female, so only look at half, so 1/2 chance.
The chance that III-1's mate is a carrier is near zero. So we make him normal. XY (normal Dad) * XaX (carrier mom III-1) Since we know its a boy you only look at half, so the chance of him having it is 1/2.
so 1/2 * 1/2= 1/4
These are two independent events, the chance the mom is a carrier * the chance the kid gets the affected X.

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aaftabsethi1  Explain it to me like i m 5 years old +5
aaftabsethi1  Also they asked about the โ€œmaleโ€ fetus . So that should be 1/2 . M i missing something? +1
tobias  @aaftabsethi1 love the reference! +1
tobias  also the answer would be 1/2 given that we know it's male but that would only be if we knew for sure that his mum was either a carrier or affected. We know his grandmother is an obligate carrier seing as the pregnant woman's brother is affected. So, the pregnant woman has 1/2 chance of being a carrier +2
jdc_md  its 1/4th because theres a 1/2 chance the mom is a carrier and theres a 1/2 chance she gives her bad gene to her son. the question stem tells you shes having a boy. if the stem did not identify the fetus gender and just asked what is the chances that a boy will have duchennes then the correct answer would be 1/8 because (1/21/21/2)= chances intereted from grandma, chance mom passes it, chance its a boy. +1
kcyanide101  @Jdc_md The answer is 1/4.... However if they didn't specify the baby's gender it will be 1/8.... 1/2 chances of the mum being a carrier times 1/4 chances of her child having it. +2

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