Genetic testing

blair424 · Thinker Joined: 07 Feb 2010 Posts: 7

I didn't realize that Marilyn sometimes recycles questions. The genetic testing question in her Feb. 7, 2010, Parade column is essentially identical to the question I submitted and she answered in her Feb. 7, 1999, column (exactly 11 years to the day!).

I actually think I've come up with a slightly better solution than Marilyn's. You see, it seems to me the weakness in her answer is the doctor knows the test results. So, if the results are negative, the doctor will be tempted to tell the patient the good news without even bothering to flip the coin. Or if the doctor does play it straight and do the coin flip, there's always the chance she might accidentally give the real test results away through her body language.

That's why I get rid of the coin flip in my solution. Instead, the doctor invites someone known to have the bad gene (the patient's parent?) to come in at the same time as the patient. Then, with both people in the same room, the doctor takes a DNA sample from each and puts them in a hat. She then pulls one sample out and destroys it and sends the other sample to the lab. Now, if the results come back negative, the patient will know her sample was the one that got tested, and therefore, she doesn't have the bad gene. And if the results come back positive, neither she nor the doctor will know whose sample got tested, and therefore, she will still have some hope that she doesn't have the bad gene.

What do you think?

supriya · Scholar Joined: 29 Nov 2009 Posts: 43

i think both of these solutions are almost similar..........n hw come we can say that one better than the other......in both patient have 50% chances of coming up clear without any genetic problem.

but ur solution is somehow better,,,in terms of ---long time assesssment because,,if it will take much more time to hear from the doctor ,so patient will fear that there r more chances of patient being positive for test.

what say ??
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from-Dr. Supriya

i believe in this proverb-
-u r as good as u think u r-

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supriya · Scholar Joined: 29 Nov 2009 Posts: 43

Why its exactly after 11 yrs,,,it cant be by chance,,,,watz the reason for it?
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from-Dr. Supriya

i believe in this proverb-
-u r as good as u think u r-

My yaho msngr id is= supriya_rastogi

supriya · Scholar Joined: 29 Nov 2009 Posts: 43

One more thing ,i wanna mention tat in the Q pt says tat she want to know if it's negative,means she want to reassure herself,,,,
But in ur solution,pt can never reassure herself even if she don't hv abnormality,,,she will always remain in doubt,,,,becoz if result even come negative she can think tat it ws another person sample,,so in this context marilyn's solution holds good....
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from-Dr. Supriya

i believe in this proverb-
-u r as good as u think u r-

My yaho msngr id is= supriya_rastogi

blair424 · Thinker Joined: 07 Feb 2010 Posts: 7

I agree with you that Marilyn's strategy and mine are equivalent because they both use a random outcome (either a coin flip or pulling a sample out of a hat) to determine whether the patient gets her real test results or a false positive. And with both strategies, if the patient is told her test results are negative, she knows she doesn't have the bad gene, and if she's told her results are positive, there's a one-in-three chance she doesn't have the gene (so she still has some hope).

What I tried to do in my strategy that Marilyn didn't is make it double-blind. As I said in my first post, the coin flip strategy assumes the doctor doesn't intentionally or unintentionally give away the patient's real test results. I don't know about you, but if I were the doctor and the results came back negative, I'd say to heck with the coin flip and I'd just tell the patient she doesn't have the bad gene. And if the results came back positive, I'd find it very hard not to show some sadness in my voice and my body language.

Of course, my strategy raises other questions, like:

- How would the doctor destroy the sample she pulled out of the hat? The patient needs to be sure only one sample gets sent to the lab. If the doctor is sneaky and sends both samples, she'll be able to figure out if the patient has the bad gene. Maybe the doctor could destroy the sample by putting it in a jar of acid.

- How can the patient be sure the doctor doesn't surreptiously obtain another DNA sample from her and send it to the lab?

- How can the patient be sure the DNA of the person known to have the bad gene doesn't have some other characteristic the doctor can use to tell which sample went to the lab. For example, if the patient is a man and the other person is a woman, and the test results come back with the bad gene and a Y chromosome, then the doctor will know the patient has the bad gene.

I'm certainly open to suggestions for improving my strategy so it could work in the real world...

supriya · Scholar Joined: 29 Nov 2009 Posts: 43

Dear,please do consider the point i 've raised in my above post.
One of main drawback of ur solution is- patient will always have a doubt in her mind that ,if the sample sent was of the other person(even when the result came negative),she can never be assure of being free of abnormal gene.
we cannot hide the thing from patient that we have taken sample from you n from one other person too n so we can't get the doubt out of patient mind that the negative result comes due to sending of other person sample.
_________________
With Regards
from-Dr. Supriya

i believe in this proverb-
-u r as good as u think u r-

My yaho msngr id is= supriya_rastogi

blair424 · Thinker Joined: 07 Feb 2010 Posts: 7

The other sample is taken from someone known to have the bad gene, so if that sample is the one sent to the lab, the lab sends back a positive test result.

So, there are four possibilities:

1. The patient has the bad gene and her sample is the one sent to the lab. The test results come back positive.

2. The patient doesn't have the bad gene and her sample is the one sent to the lab. The test results come back negative.

3. The patient has the bad gene but the sample sent to the lab is the one from the person known to have the bad gene. The test results come back positive.

4. The patient doesn't have the bad gene but the sample sent to the lab is the one from the person known to have the bad gene. The test results come back positive.

In only possibility #2 do the test results come back negative and the patient finds out she doesn't have the bad gene. In the other three possibilities, the test results come back positive, but the patient still has the hope that possibility #4 is the one that happened.

Now, consider Marilyn's four possibilities (where the doctor tells the patient the real test results if the coin flip is heads, but says the test results were positive if the coin flip is tails):

1. The patient has the bad gene and the coin flip is heads. The doctor tells the patient the test results were positive, which they were.

2. The patient doesn't have the bad gene and the coin flip is heads. The doctor tells the patient the test results were negative, which they were.

3. The patient has the bad gene and the coin flip is tails. The doctor tells the patient the test results were positive, which they were.

4. The patient doesn't have the bad gene and the coin flip is tails. The doctor lies and tells the patient the test results were positive even though they were negative.

Once again, in only possibility #2 does the doctor tell the patient the test results were negative and the patient finds out she doesn't have the bad gene. In the other three possibilities, the doctor tells the patient the test results were positive, but the patient is still left with the hope that possibility #4 happened and the doctor is lying.

supriya · Scholar Joined: 29 Nov 2009 Posts: 43

i thought in ur solution,,probability of getting the negative result lesser than that of marilyn's solution.......is n't it ?
_________________
With Regards
from-Dr. Supriya

i believe in this proverb-
-u r as good as u think u r-

My yaho msngr id is= supriya_rastogi

blair424 · Thinker Joined: 07 Feb 2010 Posts: 7

I think as long as the odds of pulling either sample out of the hat is 50% (just like a coin flip) then both strategies will produce a negative test result 50% of the time when the patient doesn't have the bad gene.

supriya · Scholar Joined: 29 Nov 2009 Posts: 43

But the thing is,one of the sample is definitely positive all the time.
_________________
With Regards
from-Dr. Supriya

i believe in this proverb-
-u r as good as u think u r-

My yaho msngr id is= supriya_rastogi

robert 46 · Intellectual Joined: 18 Jun 2007 Posts: 1194

blair424 · Thinker Joined: 07 Feb 2010 Posts: 7

robert 46 -

Thanks for correcting my description of Marilyn's solution.

A lot of times, the premise for a puzzle is childish (e.g., an island where one tribe always tells the truth and another always lies). But that doesn't mean the puzzle can't be interesting. If you don't like the genetic testing premise, you can come up with some other one, like for instance, a person who has a sealed envelope with a playing card in it, and if the card is an ace, she wants to know that, but if the card is something else, she doesn't.

supriya -

Yes, the sample taken from the person known to have the bad gene will produce a positive test result, but only if it is the one sent to the lab. Remember, the doctor takes a sample from both the patient and the person known to have the bad gene, but randomly picks only one to send to the lab. The other sample is destroyed. So, if the test results come back negative, the patient will know her sample was the one that went to the lab. But if the test results come back positive, the patient can still hope her sample didn't have the bad gene but wasn't the one sent to the lab.

supriya · Scholar Joined: 29 Nov 2009 Posts: 43

look dear,,,,,,,suppose patient don't found to have any genetic abnormality and her test would have been negative,,,,,,,,,,,but her sample was already destroyed by the doctor and in that place sent the sample of the person who definitely have abnormality n positive result...........

so see ur solution is n't fullfilling the requirements of puzzle because in this case patient even don't have any abnormality but still she have to be remain in doubt......whearas patient demand that she should know the result if it is negative...

i hope ,now u got my point......

thanks
_________________
With Regards
from-Dr. Supriya

i believe in this proverb-
-u r as good as u think u r-

My yaho msngr id is= supriya_rastogi

blair424 · Thinker Joined: 07 Feb 2010 Posts: 7

I think I get your point if you mean that neither Marilyn's strategy nor mine actually solves the problem, since the patient wants to know if she doesn't have the bad gene, but in both strategies, there's a 50% chance she won't find out if her test results are negative.

supriya · Scholar Joined: 29 Nov 2009 Posts: 43

but dear in one of solution if the sample of patient is once detroyed thr will be ,no furthur way or any hope to get the truth back,,,,,,,,,,,is n't it?

i am not saying that ur solution is n't good,,,in my view these both solutions r almost equivalent........both of these solutions r perfect at their place..
_________________
With Regards
from-Dr. Supriya

i believe in this proverb-
-u r as good as u think u r-

My yaho msngr id is= supriya_rastogi