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#1
04-03-2010
 USMLE Forums Guru Steps History: Not yet Posts: 316 Threads: 68 Thanked 421 Times in 118 Posts Reputation: 441
Genetic Probabilities?

Waardenburg syndrome is an autosomal dominant condition that accounts for 1.4% of cases of congenital deafness. In addition to deafness, patients with this condition have an atypical facial appearance, including lateral displacement of the inner canthi, hypertelorism ,white forelock and white patches of skin on the ventral midline . A mother has Waardenburg syndrome, her husband is unaffected, and they plan to have a family with three children. What is the probability that one of the three children will be affected?
A. 1/8
B. 1/4
C. 3/4
D. 1/3
E. 1/2
 The above post was thanked by: wahaha007 (04-03-2010)

#2
04-03-2010
 USMLE Forums Newbie Steps History: Not yet Posts: 4 Threads: 0 Thanked 11 Times in 2 Posts Reputation: 21

Every time they will have a child they have 50 percent chance to have an affected one.as it is an autosomal dominant pattern.
simple Mendelian inheritance
#3
04-03-2010
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 187 Threads: 4 Thanked 408 Times in 130 Posts Reputation: 438

The probability for EACH child, according to Mendelian inheritance, is indeed 1/2, but the question asks what is the probability that ONE out of THREE children is affected. In other words, what is the chance that an independent ratio is repeated three times, just like when a fellow tosses a coin 3 times and the result is either heads or tails (50% = 1/2 for each).

So, the correct answer is 1/2 x 1/2 x 1/2 = 1/8 (choice A).
 The above post was thanked by: raheel_memon (04-05-2010), Seetal (04-03-2010), wahaha007 (04-03-2010)

#4
04-03-2010
 USMLE Forums Master Steps History: Step 1 Only Posts: 919 Threads: 134 Thanked 1,406 Times in 390 Posts Reputation: 1429

yes yes A
#5
04-03-2010
 USMLE Forums Guru Steps History: Not yet Posts: 316 Threads: 68 Thanked 421 Times in 118 Posts Reputation: 441

NO WRONG!!! think again guys

anth u r on da ryt track think a bit more.... then will i post da ans
 The above post was thanked by: fauzan (04-07-2010)
#6
04-03-2010
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 187 Threads: 4 Thanked 408 Times in 130 Posts Reputation: 438
Total recall!

Ok, let's consider it another way. If H stands for healthy children and W stands for children affected by Waardenburg syndrome, then the possible makeups are as follows:

HHH (all children are healthy)
WHH (1 child is affected)
HWH ( >> )
HHW ( >> )
WWH (2 children are affected)
WHW ( >> )
HWW ( >> )
WWW (all 3 children are affected)

So, out of 8 possible outcomes, there are three among them satisfying the condition in question, i.e. the possibility that EXACTLY one of the three children is affected is 3/8. What the h*** am I doing wrong again
 The above post was thanked by: doctorF (04-03-2010), Seetal (04-03-2010)
#7
04-03-2010
 USMLE Forums Guru Steps History: Not yet Posts: 316 Threads: 68 Thanked 421 Times in 118 Posts Reputation: 441

u r ryt ath.

probabilities that one of the three children is affected are:

so there are three different orders of birth

abnormal normal normal has a probab of 1/2 x 1/2 x 1/2 = 1/8
normal abnormal normal has a probab of 1/2 x1/2 x1/2 = 1/8
normal normal abnormal has a probab of 1/2 x 1/2 x 1/2 = 1/8

So the probability of 1 abnormal and 2 normal is the sum of the probabilities of these 3 possible orders

1/8 + 1/8 + 1/8 = 3/8 which is approximately 1/3

ans is 1/3 D
 The above post was thanked by: ath.pantelis (04-03-2010), fauzan (04-07-2010), Seetal (04-03-2010)
#8
04-03-2010
 USMLE Forums Master Steps History: Step 1 Only Posts: 919 Threads: 134 Thanked 1,406 Times in 390 Posts Reputation: 1429

i dont like this question!!

so lemmie get this straight,, everytime i come a across a question like this i need to think about how many children they WANT (whether it is 2 ,3 or 4), and then each of them have a 1/2 probability (for autosomal dominant), and then times it with the probability of the chances that they will have one affected child and 2 none (in this case obviously 3 times) so 3/8 approximates 1/3.. am i making sense?
#9
04-04-2010
 USMLE Forums Guru Steps History: Not yet Posts: 316 Threads: 68 Thanked 421 Times in 118 Posts Reputation: 441

actually this is a stupid question in one of the banks i was going thruu... so i put it here... but yes genetics questions are kind of confusing for most
 The above post was thanked by: fauzan (04-07-2010)
#10
04-04-2010
 USMLE Forums Master Steps History: Step 1 Only Posts: 919 Threads: 134 Thanked 1,406 Times in 390 Posts Reputation: 1429
yeah

they did mention somewhere that beh.science is kinda difficult for most imgs. i bought this high-yield behavioral science book and i intend to do it many times!! im gonna get this and get it good.
#11
04-05-2010
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 187 Threads: 4 Thanked 408 Times in 130 Posts Reputation: 438

Although HY Behavioral Science is a very concise, well-organized book, it is not enough for the USMLE, at least not for Step 1. I don't say that I studied sth more than Kaplan, FA & HY, as well as the high yield topics found in UWorld, but the questions that appeared in the actual exam were kinda different, out of the ordinary (as usual)!!!
#12
04-05-2010
 USMLE Forums Master Steps History: Step 1 Only Posts: 919 Threads: 134 Thanked 1,406 Times in 390 Posts Reputation: 1429
thank u ath.pantelis

well in that case ill master the material that i have and pretend like im a GP when i answer beh.sci questions. ...that way ill have to apply logic based on what ive read of course.
#13
04-05-2010
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 187 Threads: 4 Thanked 408 Times in 130 Posts Reputation: 438

That's right! And it's true for EVERY USMLE question... When every piece knowledge seems to be non-applicable or even deleted, then the last resort is your logic and your powerful clinical sensorium!

 Tags Genetics-, Step-1-Questions

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