hey guys, confused about the following question from usmle.org. does this patient's mother have dmd? if so, shouldn't 'c' be the answer? (the answer is e) So are we always uncertain of carrier status of a woman because of random x inactivation? If so, how do we know that when a patient has an x-linked recessive disease, the chance that his healthy sister's child having the disease is 1/8 (where we assume that the sister has 1/2 chance of being a carrier).
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Step I genetics/inheritance question
Started by dontwantaids
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I can't answer things super great, but I can do a little bit. When it comes to questions like these (that deal with recessive inheritance and extended family members), you assume some things and don't assume other things. Like with the mom, you can assume that she is a carrier for DMD because of her above normal CK and the fact that her son (pt's brother) had DMD. And if we did a punnet square, she would be Xx, hetero for it. We assume dad is fine because if he had it, he would of had DMD and the stem states that it was her maternal uncle (related to mom again).
So, since we know mom is a carrier, it's 50/50 if mom will pass on that "x" to the daughter. Leaving E as the answer. We can't say C because it's a 50/50 of being X vs x.
For the rest of your questions, to a point, you don't know when a patient has a recessive disorder unless you're told it specifically or given clues that really point to it being true and then you might get asked the percentage. But most of the time, it ends up being math. 1/2 for maybe being a carrier X 1/2 for passing on the trait x 1/2 for male gender and sometimes 2/3 is in there as well, depending on what the question is asking. For questions like those, I really recommend googling usmle Cystic fibrosis probability questions. Seeing examples and explanations of those types of questions I think will really help.
So, since we know mom is a carrier, it's 50/50 if mom will pass on that "x" to the daughter. Leaving E as the answer. We can't say C because it's a 50/50 of being X vs x.
For the rest of your questions, to a point, you don't know when a patient has a recessive disorder unless you're told it specifically or given clues that really point to it being true and then you might get asked the percentage. But most of the time, it ends up being math. 1/2 for maybe being a carrier X 1/2 for passing on the trait x 1/2 for male gender and sometimes 2/3 is in there as well, depending on what the question is asking. For questions like those, I really recommend googling usmle Cystic fibrosis probability questions. Seeing examples and explanations of those types of questions I think will really help.
thanks for the great explanation. i didn't know that CARRIERS of dmd might have above normal ck; i was thinking that the mother was homozygous and thus HAD the disease and thus high ck, but it makes sense now.
