Bio Question Wording

[McNCheese]

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I'm wondering how you guys would interpret a question like this -

"What is the probability that a woman whose father has hemophilia(sex-linked recessive), and who marries a normal man, will have an affected son?"

This is a question from BR Bio (section II, number 12).

I said 25%, because I took into account that she would be having a son versus a daughter. BR says that we only care about the sons, so the answer should be 50%.

At first I kind of shrugged it off, because I decided I didn't agree with BR. But I thought I'd see how other people would interpret it, in case I'm just being stubborn.

 

[dragon529]

why would you account for both the son and daughter if they only asked for the "affected son"?

 

[thebillsfan]

hm. i would count the son too. if the questin said: if the couple had a son, what is the probability that he would be affected? then id agree with BR. but I do think it should be 25%. youre only caring about the son, but the question does not state that there IS going to be a son

 

[Charles_Carmichael]

Draw out a punnett square. She can have two sons. Out of those two sons, one has a chance of being affected by hemophilia. So, 1/2 = 50%. You don't care about the daughters because they need two copies of the gene in order to be affected by hemophila. But since the mom is only a carrier and the dad is normal, the girls can only get one copy at most. This means they are carriers, but they are not affected by hemophilia. Hope this help.

 

[Charles_Carmichael]

hm. i would count the son too. if the questin said: if the couple had a son, what is the probability that he would be affected? then id agree with BR. but I do think it should be 25%. youre only caring about the son, but the question does not state that there IS going to be a son

The question asks for a probability. I doubt many population genetics question will absolutely say whether couple has a child of a specific gender. Calculating the probability involves drawing out the punnett square and doing things like the way I explained in my previous post.

 

[McNCheese]

hm. i would count the son too. if the questin said: if the couple had a son, what is the probability that he would be affected? then id agree with BR. but I do think it should be 25%. youre only caring about the son, but the question does not state that there IS going to be a son

This was more what I was asking about, I get the punnett square and all that. I guess BR's answer kind of makes sense though, because like Kaushik said, the girls at most would be carriers.

I guess the reason I interpret it the other way is because in my genetics course, they would get all tricky, and you'd have to pay attention to whether they were saying "what is the chance that their son is affected" or "what is the chance they will have an affected son".

Thanks for your responses.

 

[zkbd9]

The answer should be 50%. If a woman's father has hemophilia then the woman will be a carrier because she will get a normal X chromosome from her mother and must get the other X (hemophilia) from her father. Her male offspring will only have one X chromosome which must come from the mother, as the Y comes from the father. The mother has two X chromomsomes: one normal and one carrying the recessive hemophilia gene. Of the maternal chromosomes, it is a 50-50 chance that either chromosome will be inherited by a son. Hope that helped.

 

[ThirdEye]

The wording is B.S. It can be interpreted either way. They should have said, "What is the probability that a son will be affected?"

Asking, "What is the probability that they will have an affected son" can too easily imply that you should factor in the probability of having a son over a daughter. If wording like this shows up on my MCAT then I'm going to have to put my fist through the monitor.

 

[wanderer]

It's worded vaguely, but I think the more proper answer is 25% since you aren't told that they have a son.

 

[Omni]

Yeah, I would have said 25% too. That definitely is vague.

 

[shahcoco]

bump. I too said 25% because of the wording.

 

[neurodoc]

If the question asked "what is the probability that their next child will have hemophilia, the correct answer would be 25%...assuming that they
have a 50% chance their next child will be a boy, and if so, he will have a 50% chance of having the allele... and .5x.5=.25. So I agree the question
is unnecessarily ambiguous.

 

[Arayh]

I agree that the phrasing is certainly not the best for this question, and probably will be phrased better on the MCAT.

 

[inasensegone]

The wording is B.S. It can be interpreted either way. They should have said, "What is the probability that a son will be affected?"

Asking, "What is the probability that they will have an affected son" can too easily imply that you should factor in the probability of having a son over a daughter. If wording like this shows up on my MCAT then I'm going to have to put my fist through the monitor.

Hahaha. I realize this is almost 5 years old, but this brought some levity to my studying in the wee hours of the morning.