TBR - Genetics Question

[kevindurant]

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this is another example of how TBR CBTs have flawed questions.

"Based on the pedigree shown in Figure 1, what is the probability that the oldest sib is a heterozygote for the PKU trait?"

it gives you 2 generations of a tree for PKU. PKU it says is an autosomal recessive disease. the parents are unaffected, and the 4 siblings are the following:
from left to right with X being they have PKU.
0 X X O
it says the siblings are left to right oldest to youngest. so the individual we are dealing with is the unaffected one on the very left.

since the parents are unaffected and half their kids have PKU, then both parents are heterozygous (Pp x Pp). so each child has 25% PP, 50% Pp, 25% pp. i chose 1/2 and got it wrong. it says the answer is 2/3 because you know it doesn't have the "pp" since the oldest sib is not recessive.

that question is poorly worded. you can try to claim "well the mcat is tricky and you have to watch out," but guess what, the real mcat isn't that lame. the question should say they only want you to take into account it being unaffected. i know the pedigree says it is unaffected, but the question doesn't tell you to neglect the 25% chance that it could have been recessive.

i don't know. this question is crappy. TBR is full of bad questions like this, not to mention bad typos that mess up given equations.

 

[niranjan162]

it is sucky but i remember getting that condition from genetics class. I wouldnt be too surprised if that showed up on the MCAT, they are tricky SOBs too.

 

[bluemonkey]

this is another example of how TBR CBTs have flawed questions.

"Based on the pedigree shown in Figure 1, what is the probability that the oldest sib is a heterozygote for the PKU trait?"

it gives you 2 generations of a tree for PKU. PKU it says is an autosomal recessive disease. the parents are unaffected, and the 4 siblings are the following:
from left to right with X being they have PKU.
0 X X O
it says the siblings are left to right oldest to youngest. so the individual we are dealing with is the unaffected one on the very left.

since the parents are unaffected and half their kids have PKU, then both parents are heterozygous (Pp x Pp). so each child has 25% PP, 50% Pp, 25% pp. i chose 1/2 and got it wrong. it says the answer is 2/3 because you know it doesn't have the "pp" since the oldest sib is not recessive.

that question is poorly worded. you can try to claim "well the mcat is tricky and you have to watch out," but guess what, the real mcat isn't that lame. the question should say they only want you to take into account it being unaffected. i know the pedigree says it is unaffected, but the question doesn't tell you to neglect the 25% chance that it could have been recessive.

i don't know. this question is crappy. TBR is full of bad questions like this, not to mention bad typos that mess up given equations.

I don't see why this is a bad question. It definitely seems like MCAT style to present a rather straightforward genetics problem that has a twist in the form of a pedigree which precludes one of the possible genotypes. The entire point of this problem is that the question shouldn't have to tell you to neglect the 25% chance. This is what the pedigree does. This isn't "tricky." It's critical thinking. If you are so unhappy with the TBR questions then why don't you work on something else. You might be in for a nasty surprise if you think the MCAT doesn't have questions like this.

 

[kevindurant]

I don't see why this is a bad question. It definitely seems like MCAT style to present a rather straightforward genetics problem that has a twist in the form of a pedigree which precludes one of the possible genotypes. The entire point of this problem is that the question shouldn't have to tell you to neglect the 25% chance. This is what the pedigree does. This isn't "tricky." It's critical thinking. If you are so unhappy with the TBR questions then why don't you work on something else. You might be in for a nasty surprise if you think the MCAT doesn't have questions like this.

yea, yea, i take it back. i was just venting when i wrote this. i agree...the mcat definitely tricks you like that. that's the only thing that makes the mcat difficult.

after finishing the TBR tests, i went back to AAMC ones and am now kicking ass! TBR made me improve my critical thinking SO MUCH. i finally keep in mind to look at every question thinking there is a trick involved. most of my mistakes before were from being tricked. i still am making some mistakes like that, but the # of them are definitely reduced. thanks TBR!

 

[iliveinmyguitar]

I got this question wrong cause I thought the answer choise was a little p- not big P-.. fml