Autosomal Dominant Disease with 80% Penetrance? - USMLE Forums
 USMLE Forums         Your Reliable USMLE Online Community     Members     Posts
 Home USMLE Articles USMLE News USMLE Polls USMLE Books USMLE Apps
 USMLE Forums Autosomal Dominant Disease with 80% Penetrance?
 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
06-01-2012
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 34 Threads: 11 Thanked 0 Times in 0 Posts Reputation: 10
Autosomal Dominant Disease with 80% Penetrance?

So I ran into this question on kaplan:

"Two parents are heterozygotes for a dominant trait that has 80% penetrance. what is the probability of producing a phenotypically normal offspring?"

The answer is 40%. this was their explanation:
*75% of the offspring will receive either one or two of the affected gene.
* Multiply this by the penetrance and you have the affected probability. 0.75*0.8: 0.6
* subtract from 1 to get the phenotypically normal kids, 1-.6: 0.4.

The problem is that they are leaving out the 25% of the kids that will not get the gene, so this was my approach:
* 25% of the offspring wont inherit the gene.
*0.75 * 0.2: will not have the disease phenotypically.
* Add both probabilities because they are independent from each other: .25+ .15: 40%.

I need to know if my approach is correct or if there is an explanation to just forget about the homozygous normal offsprings.

#2
06-01-2012
 USMLE Forums Addict Steps History: Not yet Posts: 135 Threads: 3 Thanked 73 Times in 48 Posts Reputation: 83

Your approach is correct, if you're comfortable with it, stick to it.

Theirs is also correct, since the question is asking for the probability of a phenotypically normal offspring. All they did was calculate the probability of the phenotypically affected and subtract it from 1. That'll give you the probability of the phenotypically normal. This includes the 25% who're homozygous recessive, plus the genotypically affected that won't present with the phenotype.
__________________

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
You can't confuse the confused!
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
 The above post was thanked by: JennGar (06-01-2012)
#3
06-01-2012
 USMLE Forums Scout Steps History: Step 1 Only Posts: 50 Threads: 5 Thanked 59 Times in 23 Posts Reputation: 69

Folks discussed that here
Autosomal Dominant Disease with 80% Penetrance?
 The above post was thanked by: JennGar (06-01-2012)

 Tags Genetics-

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.