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Old 06-01-2012
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Genetics 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.
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Old 06-01-2012
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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.
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Old 06-01-2012
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Folks discussed that here
Autosomal Dominant Disease with 80% Penetrance?
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