Can someone explain the answer to this genetic probability problem?

[Fanghur]

In the review section of my textbook (not for marks, don't worry. In my class marks come only from the midterm and final exam) there's a question which I'm not entirely sure how to calculate; namely because it sounds like a trick question. The first part was fairly easy: If someone with the genotype aa mates with someone with a genotype that's 0.5 Aa/0.5 AA, what is the probability that their first child will be affected (i.e. have the genotype aa). (This question is about a rare autosomal recessive allele).

 

This is the calculation (I think): P(aa) = 0.5Aa x 1.0aa x 0.5(aa) = 0.25 aa.

 

But the next part is what has me a little bit confused, because it asks what the probability that their first child would be phenotypically normal (i.e. Aa or AA). Now, logically I would think that since there's a 25% chance of their first child being affected, then by definition there would be a 75% chance of them being unaffected.

 

Am I missing something here? Because it really sounds like it's some sort of trick question.

[Ringer]

Just seems like you may be over thinking it. Probability wise the probability of A plus the probability of not A is 1. So if there is a .25 probability there is a .75 probability of anything else.

[jorden]

Your right. Your formula.was different. I simplified it using the square. You have 8 total solutions. 6 are Aa and 2 are aa. Make fractions and reduce. 75% Aa and 25% aa. So yes your right. Not really a trick question as it is throwing in another genotype to make you think about it. You can't over think in science or you end up with huge marginal errors.