1/2
If you make a punnet square with mom on the top and dad on the left side:
A a
A AA Aa
a Aa aa
You can see from this that the possible offspring will be AA, Aa, Aa, and aa. So the offspring that will have the same genotype as their parents (Aa) will occur 2/4 (or 1/2) of the time.
AA = 1/4
Aa = 2/4 or 1/2
aa = 1/4