AAMC FL 2 BB Q 31

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mgarc805
Posts: 35
Joined: Tue Jun 25, 2019 11:23 am

AAMC FL 2 BB Q 31

Post by mgarc805 » Sun Jul 28, 2019 12:09 pm

First, I don't understand if CFTR is a protein or a gene
he CFTR gene is found on an autosome. The most common mutation of the CFTR protein (ΔF508)
and again the question stem makes it look like a gene.

Second, the Question stem says that G542X is another allele of the same gene, so that would put G542X and CFTR in the same locus (assuming that CFTR is a gene). The this is what gets confusing because then they are saying that a person can be Aa and the mate can be Bb so what would be the result of that or in this case they are saying one person is G542X -/+ while the other person if CFTR+/- and that makes no sense to me in terms of what cross can happen there. Now at the end of the Question stem they say so assume that none of the parents suffer the disease which clues me that you could infer the other pair for each allele on each parent so the parent that has G542X -/+ would have a mate that is G542X -/- and CFTR +/-. the former in terms will be G542X-/+ and CFTR -/-.

overall my confusion arises from the fact that I don't understand how two different alleles are express for the same gene ( Aa X Bb)
NS_Tutor_Mathias
Posts: 264
Joined: Sat Mar 30, 2019 8:39 pm

Re: AAMC FL 2 BB Q 31

Post by NS_Tutor_Mathias » Mon Jul 29, 2019 1:08 pm

CFTR - Italics, capitals - gene
CFTR - Regular font, capitals - protein
ΔF508 - Italics, capitals - knockout at position 508 of amino acid phenylalanine


So, CFTR ΔF508 is an allele of CFTR. As is G542X (which is in actuality CFTR G542X). So these are all alleles of the same gene, meaning slightly different versions of the same gene.

In the question stem, that gives us the following:

Mother: CFTR G542X/CFTR G542X
Father: CFTR WT/CFTR ΔF508


And the question asks you simply what the chance for homozygosity for G542X is. The answer to that is clearly zero, since the offspring will receive one allele from each parent (short of some sort of viable trisomy, but that is not worth worrying about unless a passage specifically mentions it).

Now at the end of the Question stem they say so assume that none of the parents suffer the disease
I hope my clarification on nomenclature above makes it easy to see why this is a redundant statement even in the question stem. The passage tells us that only homozygotes for ΔF508 have the CF phenotype. The question already tells us that no homozygotes for CF are present, nor would it even change the question or the answer at all if they were. This is what is usually called a red herring: Something to distract you.
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