I don't understand this question. They equate carrier-status to mean affected by the allele. We know her mother is a carrier because she has elevated CK. That means the patient has a 50% chance to be a carrier since she's female. Random X-inactivation doesn't matter when talking about carrier status because both alleles have a chance to get passed on. Random X inactivation is only important for determining whether she will develop symptoms.
I guess this is why the question was thrown out? Unless I'm missing something.
Why is the answer carrier status unknown? Why can't the mother be homozygous? Duchennes is X linked recessive.
This probably has to do with Barr Bodies (FA 2021, pg 61). To paraphrase FA, penetrance and severity of diseases in XX individuals can be impacted. DMD is included in their list.
Her mom has high CK because her X inactivation is likely imbalanced to have more mutated X expressed. But note that a high CK does not equal full-blown Duchennes. It's similar to Hemophilia--women can have increased bleeding but not full hemophilia because of this phenomenon.
Her mother has some Duchenne phenotype, but we could just as easily see no phenotype in a carrier.
Further, if this woman is a carrier, her child has a 25% chance of developing DMD (50% chance of a boy, 50% of the bad X).
This woman likely has a 50% chance of being a carrier, she doesn't have DMD based on her CK and age and gender.
submitted by โbwdc(697)
DMD is X-linked. We know her mom is a carrier based on family history, supported by lab testing. But her mom has 2 X chromosomes, only one of which is mutated. There is no way to know which her daughter eventually receives and expresses by her phenotype (i.e if she is a carrier or not). Just because her CK is normal doesnโt mean she isnโt a carrierโthe phenotype of the X-linked carrier depends on X-inactivation.