NBME 24 Answers : Block 2 : #42 = A 3-week-old female newborn is brought to the ...

+6 upvote downvote
submitted by colonelred_(54),

The analysis only showed a mutation in one allele. CF is an autosomal recessive disease: the disease only manifests if there are mutations in both alleles of the CFTR gene.

If you still have 1 functional copy of the CFTR gene, you can still make the CFTR protein (the chloride channel/transporter), hence your body won’t have any issues.

This is analogous to tumor suppressor genes like Rb: so long as one of the alleles you have is functional, you can make enough of the protein to “make up” for the defective allele. If both get knocked out (Rb-/-), you lose the protection provided by the gene because now you make no protein at all.

The only thing that made sense for this question was the fact that the other allele was not included in the analysis.

charcot_bouchard OR another allele has a diff type of mutation because CF is done by like hundreds of diff type of mutation. SO the 70 types that we screened covered one type from one parent but not another that was inherited from other parent. +6

soph I put D thinking there was a mutation in another protein that interacts with CFTR....thus u dont have CF but some disease with similar phenotype. Is this wrong bc its simply not the case ?? +

nbmehelp @charcot_bouchard I think that makes more sense if I understand what you're saying- Probably had a mutation only in 1 of 2 of the same alleles in the analysis but had another mutation in 2 of 2 alleles at a different location not included in the analysis, right? +

fallot4logy CF is a rare disease , and the possibility to have a mutated gene plus a gene that its not belong to 70 most common cf mutations is extremely rare +1