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#1
04-05-2016
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 96 Threads: 26 Thanked 22 Times in 17 Posts Reputation: 32
Question about a particular Qbank question

I got the following question:

2 parents (mother is pregnant) who have cystic fibrosis in their family are concerned that there child will have CF and consult a physician. They show a pedigree, but basically the 2 parents are obligate carriers of the CF gene (heterozygous normal). They ask what's the probability that the child will be homozygous normal genotype?

I chose 1/4. 1/4 chance he'll be homozygous normal, 2/4 chance he'll be heterzygous normal (carrier), and 1/4 chance homozygous affected. But the answer was 1/3, saying that you don't count the 1/4 chance of homozygous affected. Keep in mind this is a fetus, so we don't know if he's affected or not, so I don't know why they want us to remove the possibility of being affected.

I'm worried on the USMLE I'll get a question like this, but not sure what the question is asking exactly. Shouldn't they have instead worded it like, What's the probability that the fetus will be unaffected, AND have the homozygous normal genotype?

#2
04-06-2016
 USMLE Forums Veteran Steps History: 1+CK+CS+3 Posts: 255 Threads: 26 Thanked 85 Times in 71 Posts Reputation: 95

First off, the question may have been worded differently, but your issue with it is irrelevant. I believe the question is asking for the odds... which is the probability of an event occurring vs. the probability of it not occurring... p/(1-p)

1 child out of 4 is likely to be homozygous normal genotype
3 children out of 4 are not likely to behomozygous normal genotype

1/4 divided by 3/4 is.... 1/3

Thus, probability that the child will be homozygous normal genotype is 1/3.
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#3
04-06-2016
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 96 Threads: 26 Thanked 22 Times in 17 Posts Reputation: 32

So what if a question ask what's the chance the child will be homozygous affected? Or even if it says what's the chance the child will be affected with the disease? Surely you won't do (p/1-p) again and say it's a 1/3 chance, when we all know there's a 1/4 chance of a child being affected by an AR disease?

My main issue is I'm not sure how to tell exactly in which way do they want me to yanswer the question. With a punnet square, or the way you described, more like a Behavioral Sciences question than Genetics.
#4
04-06-2016
 USMLE Forums Veteran Steps History: 1+CK+CS+3 Posts: 255 Threads: 26 Thanked 85 Times in 71 Posts Reputation: 95

There is a difference between "chance" and "odds"...

There is a 1/4 chance of the child having a homozygous normal genotype, while the odds of the child having a homozygous normal genotype is 1/3.
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#5
04-06-2016
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 96 Threads: 26 Thanked 22 Times in 17 Posts Reputation: 32

Quote:
 Originally Posted by Kruno There is a difference between "chance" and "odds"... There is a 1/4 chance of the child having a homozygous normal genotype, while the odds of the child having a homozygous normal genotype is 1/3.
Thank you. I PM'd you with the exact img of the question to confirm.
#6
04-06-2016
 USMLE Forums Veteran Steps History: 1+CK+CS+3 Posts: 255 Threads: 26 Thanked 85 Times in 71 Posts Reputation: 95

you wrote the question wrong in your thread...... i was going to ask if its a uworld question or not, because they do not get the wording all mixed up. here i assumed that since the answer was 1/3, you didn't write the question correctly and that they were asking for the odds... a question i have seen in a qbank before.

however, it was much more straightforward... it asked for the probability that a normal child will have a homozygous normal genotype.
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#7
04-06-2016
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 96 Threads: 26 Thanked 22 Times in 17 Posts Reputation: 32

Quote:
 Originally Posted by Kruno you wrote the question wrong in your thread...... i was going to ask if its a uworld question or not, because they do not get the wording all mixed up. here i assumed that since the answer was 1/3, you didn't write the question correctly and that they were asking for the odds... a question i have seen in a qbank before. however, it was much more straightforward... it asked for the probability that a normal child will have a homozygous normal genotype.
OMG!

Think I should just kill myself right now. Can't believe I missed that.

Out of curiosity, do you remember what the question was that you saw? I'm curious now, don't want to be tricked in the future.
#8
04-06-2016
 USMLE Forums Veteran Steps History: 1+CK+CS+3 Posts: 255 Threads: 26 Thanked 85 Times in 71 Posts Reputation: 95

i cant remember where i saw it, but i wouldnt worry about it. usmle questions generally dont try to trick you. if they ask for the odds, they will probably make it clear.
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