1/2 x 1/2 parents = 1/4 risk for a child to have a disease and the same 1/4 chance to be homozygous normal 1/2 to be a carrier I think.

 

you fell for the trick....simple question but you gotta figure what exactly they're asking...I fell for it too.

 

Since he's unaffected then he can either be a carrier or a homozygous normal.

So his chance of being homozygous normal is 1 in 3. Because 1 in 4 is homozygous affected, 2 in 4 carriers, and 1 in 4 homozygous normal.

 

I hate genetics.

the guys already have a child...

 

1/3 hence, 33%

 

Yeah, its one in three.
Aa and Aa will result in AA Aa Aa & aa.
Here, we already know the child is normal so aa is out.
Therefore ,out of the three left....the probability of the child being homozygous normal(AA) is 1 out of 3.