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Q:A 28y woman and a 25y man present fore genetic counseling. Both are white and have one sibling affected with cystic fibrosis. The most appropriate method to assess the risk of transmitting cystic fibrosis to a potential child would be:

A. biochemical testing
E. Western blot analysis

The correct answer is D, but I think all the 4 answers work. A and E could examine the defect protein, B and D could examine the defect DNA. Need some help here, thank you!!!


Cystic fibrosis is an autosomic recessive disease. The question seems to be talking about the most common mutation in Caucasians, and so the chance or risk of transmitting the disease is already known (1/4 or 25%); especially if they already have a sibling with the disease. The usefulness of doing genetic testing would be to find out in an ammiocentesis or chorionic villous sampling if an embryo or fetus carries both copies.

Yes, you're right. All methods could potentially work as long as the expression of the Cystic Fibrosis Channel is abundant in the cells obtained from the fetus.  For Western blot to work you need a good antibody capable of distinguishing the wildtype vs. the mutated channel. For biochemical testing, some specific assay would need to be devised, possibly patch clamp to assess the electrical current of the channel.  FISH is good to detect macromutations like a piece of chromosome missing or in excess.

The best answer is PCR which you would apply to any cell since you only need DNA. By designing appropriate amplimers you could tell you if the most common triple nucleotide mutation is missing or present (F508). You would design one of the primers (either forward or reverse) to have in its most 3' end the cystic fibrosis triplet. The fetus with both wildtype copies would get good amplification, say 2x. The fetus with one good copy and a bad one would get amplification about 1x. The fetus with the disease would get no amplification of a band of the right size.

AAAAAAAAAATTT..............    (primer) amplification (.....)
AAAAAAAAAATTTNNNNN   (target region - template)

AAAAAAAAAATTT                     (no amplification)
AAAAAAAAAAxxxNNNNN        (xxx = missing triplet, no annealing)

Something like this. I'm not sure if this is the way it works, possibly they have something more sofisticated, like Taqman probes to detect single nucleotide polymorphisms.
'hope it helps.




It's a nicely-explained answer!
So you mean since we have already known the most common mechanism of cystic fibrosis, PCR is the relatively most convenient test, thus the best answer~
Thank you, Xenopus!!!

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