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#1
02-13-2011
 USMLE Forums Guru Steps History: Step 1 Only Posts: 487 Threads: 95 Thanked 1,466 Times in 358 Posts Reputation: 1476
The probability that these genes transfer together?

Genes on the bacterial chromosome have the following linkages in conjugal transfer: x and y, 25% of the time; y and z, 50% of the time. If the gene order is x-y-z, approximately what percentage of the time will x and z be transferred together?

A. 1% of the time
B. 5% of the time
C. 13% of the time
D. 20% of the time
E. 40% of the time
 The above post was thanked by: DoctorNo (02-14-2011), ibukun (02-13-2011), Mondoshawan (02-13-2011)

#2
02-13-2011
 USMLE Forums Master Steps History: 1+CK+CS Posts: 590 Threads: 31 Thanked 1,233 Times in 411 Posts Reputation: 1257
C.

My guess is C. 13% of the time

Chance that a Y will come with a Z (.5) * chance that an X will come with that Y (.25)

.25 * .50 = .125 ≈ 13%
 The above post was thanked by: aktorque (02-14-2011)
#3
02-13-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,684 Threads: 213 Thanked 1,600 Times in 660 Posts Reputation: 1610
Not sure

Im not sure how to answer this question
but i was thinking along the lines of

(0.25+0.50) - (0.25*0.50) which equals 62.5

this gives probability of not mutually exclusive events.
Not sure, im waiting for the answer and an explanation!
 The above post was thanked by: aktorque (02-14-2011), Mondoshawan (02-14-2011)

#4
02-13-2011
 USMLE Forums Master Steps History: 1+CK+CS Posts: 590 Threads: 31 Thanked 1,233 Times in 411 Posts Reputation: 1257

Quote:
 Originally Posted by Hope2Pass Im not sure how to answer this question but i was thinking along the lines of (0.25+0.50) - (0.25*0.50) which equals 62.5 this gives probability of not mutually exclusive events. Not sure, im waiting for the answer and an explanation!

Doesn't that give the probability of one of two events happening or both (genes transferred ∈ {x,y} ∪ {y,z} ∪ {x,y,z})?

The events for which the probabilities are given aren't mutually exclusive; however, they are also not separate, since x, y, and z are on the same chromosome. {x,y,z} is the only set that could include both x and z.

I was thinking of the probabilities as approximating synteny; that is to say, since y and z together is twice as probable as x and y, then y and z must be about half the distance apart:

------x------y---z----------

So the probability that the whole string would be transferred would approximate the probability of the x-z synteny, which would be the inclusive probability of both the x-y and y-z syntenies. Which, I think, is the product of the two.
 The above post was thanked by: aktorque (02-14-2011), Hope2Pass (02-14-2011)
#5
02-14-2011
 USMLE Forums Guru Steps History: Step 1 Only Posts: 487 Threads: 95 Thanked 1,466 Times in 358 Posts Reputation: 1476

Quote:
 Originally Posted by Mondoshawan My guess is C. 13% of the time Chance that a Y will come with a Z (.5) * chance that an X will come with that Y (.25) .25 * .50 = .125 ≈ 13%

Very good.

Correct Ans is C) 13%
 The above post was thanked by: DoctorNo (02-14-2011), Hope2Pass (02-14-2011), Mondoshawan (02-14-2011)

 Tags Biostatistics-Epidemiology, Step-1-Questions

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