genetic Q - USMLE Forums
 USMLE Forums         Your Reliable USMLE Online Community     Members     Posts
 Home USMLE Articles USMLE News USMLE Polls USMLE Books USMLE Apps
 USMLE Forums genetic Q
 Register FAQs Members List Search Today's Posts Mark Forums Read

 USMLE Step 1 Forum USMLE Step 1 Discussion Forum: Let's talk about anything related to USMLE Step 1 exam

#1
09-07-2011
 USMLE Forums Addict Steps History: CK Only Posts: 105 Threads: 19 Thanked 69 Times in 32 Posts Reputation: 79
genetic Q

A man ans women are both affectedby an autosomal dominant disorder that has 80 % penetrance. They both heterozygotes for the disease-causing mutation. What is the probabiity that they will produce phenotypically normal offspring?

a. 20%
b.25%
c.40%
d.60%
e.80%
f.non of the above
 The above post was thanked by: dr_lizard (09-07-2011)

#2
09-07-2011
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 69 Threads: 4 Thanked 76 Times in 21 Posts

It would be 75% chance of getting at least one diseased allele, then that times 80% penetrance = 60% disease risk, then C
#3
09-07-2011
 USMLE Forums Addict Steps History: Not yet Posts: 189 Threads: 17 Thanked 95 Times in 58 Posts Reputation: 105

Aa - Aa ==> AA(1/4x20%x20%)+Aa(1/2x20%)+aa(1/4)=36%
is it right?
__________________
nothing is impossible!

#4
09-07-2011
 USMLE Forums Master Steps History: Step 1 Only Posts: 567 Threads: 25 Thanked 524 Times in 263 Posts Reputation: 534

Interesting, lets do this ...

* A a
*A AA Aa
*a Aa aa

assuming that

AA = Normal
Aa = Heterozigous
aa = Affected

Remember here, we just need 1 gene to be affected due the fact that is AD. so the Normal Calculation is that "75%" of this Disease will be afected the calculation is like this 75 Affected by 80% penetrance is 60 affected so we sustract 60-100 = 40 !

In a normal 100% penetrance 75 would be affected, but this got 80 %, so we have to do the math for that.

75 Affected MINUS (-) 80 = 60 % will be affected in 80% of penetrance... so doing the subtraction is 40 for NORMAL.

So...

Answer is C. Corresponding the NORMAL Child
Answer D correspond the Affected Childrens...
Answer E correspond to the penetrance of the disease.
Answer B and A wrong calculations...

Hope it helps...
__________________
Nothing is Impossible, the Word Itself says "I'm Possible"
 The above post was thanked by: dr_lizard (09-07-2011)
#5
09-07-2011
 USMLE Forums Addict Steps History: Not yet Posts: 189 Threads: 17 Thanked 95 Times in 58 Posts Reputation: 105

Quote:
 Originally Posted by rulz Interesting, lets do this ... * A a *A AA Aa *a Aa aa assuming that AA = Normal Aa = Heterozigous aa = Affected Remember here, we just need 1 gene to be affected due the fact that is AD. so the Normal Calculation is that "75%" of this Disease will be afected the calculation is like this 75 Affected by 80% penetrance is 60 affected so we sustract 60-100 = 40 ! In a normal 100% penetrance 75 would be affected, but this got 80 %, so we have to do the math for that. 75 Affected MINUS (-) 80 = 60 % will be affected in 80% of penetrance... so doing the subtraction is 40 for NORMAL. So... Answer is C. Corresponding the NORMAL Child Answer D correspond the Affected Childrens... Answer E correspond to the penetrance of the disease. Answer B and A wrong calculations... Hope it helps...
tanx, but which part of my answer's logic is incorrect?
__________________
nothing is impossible!
#6
09-07-2011
 USMLE Forums Master Steps History: Step 1 Only Posts: 567 Threads: 25 Thanked 524 Times in 263 Posts Reputation: 534

Quote:
 Originally Posted by dr_lizard tanx, but which part of my answer's logic is incorrect?
I didnt understand your answer. Could you please explain it to me, what did you do on it ? but im pretty sure my answer is the right one, if you have any hard time understand it let me know, i help you out ... =) That's the right way to do that calculation...
__________________
Nothing is Impossible, the Word Itself says "I'm Possible"
#7
09-07-2011
 USMLE Forums Addict Steps History: CK Only Posts: 105 Threads: 19 Thanked 69 Times in 32 Posts Reputation: 79

yes you r right, the answer is 40%
thank you for the explanation
#8
09-07-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 623 Threads: 111 Thanked 424 Times in 264 Posts Reputation: 434

Quote:
 Originally Posted by laurier A man ans women are both affectedby an autosomal dominant disorder that has 80 % penetrance. They both heterozygotes for the disease-causing mutation. What is the probabiity that they will produce phenotypically normal offspring? a. 20% b.25% c.40% d.60% e.80% f.non of the above
its b,25% chance to produce normal offspring.
#9
09-07-2011
 USMLE Forums Master Steps History: Step 1 Only Posts: 567 Threads: 25 Thanked 524 Times in 263 Posts Reputation: 534

Quote:
 Originally Posted by laurier yes you r right, the answer is 40% thank you for the explanation
You welcome =)
__________________
Nothing is Impossible, the Word Itself says "I'm Possible"
#10
09-07-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 623 Threads: 111 Thanked 424 Times in 264 Posts Reputation: 434

Quote:
 Originally Posted by rulz I didnt understand your answer. Could you please explain it to me, what did you do on it ? but im pretty sure my answer is the right one, if you have any hard time understand it let me know, i help you out ... =) That's the right way to do that calculation...
#11
09-07-2011
 USMLE Forums Master Steps History: Step 1 Only Posts: 567 Threads: 25 Thanked 524 Times in 263 Posts Reputation: 534

Quote:
 Originally Posted by aknz Please explain it
First, read the answer above and let me know which part u dont understand... pls. =) I detailed the answer...
__________________
Nothing is Impossible, the Word Itself says "I'm Possible"
 The above post was thanked by: aknz (09-07-2011)
#12
09-07-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 623 Threads: 111 Thanked 424 Times in 264 Posts Reputation: 434

Quote:
 Originally Posted by rulz First, read the answer above and let me know which part u dont understand... pls. =) I detailed the answer...
Thanks I understand it

AD disease,for 100% penetrance, 75% will be affected.
For 80% penetrance,60% will be affected (did calculation for that)
#13
09-07-2011
 USMLE Forums Master Steps History: Step 1 Only Posts: 567 Threads: 25 Thanked 524 Times in 263 Posts Reputation: 534

Quote:
 Originally Posted by aknz Thanks I understand it AD disease,for 100% penetrance, 75% will be affected. For 80% penetrance,60% will be affected (did calculation for that)
Exactly!! you got it.. =)
__________________
Nothing is Impossible, the Word Itself says "I'm Possible"
 The above post was thanked by: aknz (09-07-2011)
#14
09-07-2011
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 69 Threads: 4 Thanked 76 Times in 21 Posts

Quote:
 Originally Posted by dr_lizard tanx, but which part of my answer's logic is incorrect?
I guess you are overthinking this. You are assuming that the child has 80% penetrance for each diseased allele he/she carries, in other words that a child with AA genotype has a higher than 80% chance of expressing the phenotype. That is why in your calculation you used 1/4x20%x20%. If you simply assume that the risk is still 80% even with the AA genotype, then the math does come to 40% = .25 x .2 + .5 x .2 + .25
 The above post was thanked by: dr_lizard (09-07-2011)

Message:
Options

## Register Now

In order to be able to post messages on the USMLE Forums forums, you must first register.
User Name:
Medical School
Choose "---" if you don't want to tell. AMG for US & Canadian medical schools. IMG for all other medical schools.
 AMG IMG ---
USMLE Steps History
What steps finished! Example: 1+CK+CS+3 = Passed Step 1, Step 2 CK, Step 2 CS, and Step 3.

Choose "---" if you don't want to tell.

 Not yet Step 1 Only CK Only CS Only 1 + CK 1 + CS 1+CK+CS CK+CS 1+CK+CS+3 ---
Favorite USMLE Books
 What USMLE books you really think are useful. Leave blank if you don't want to tell.
Location
 Where you live. Leave blank if you don't want to tell.

## Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.