It's easier than it seems!
Do the Punnet square.
Count how many times a given genotype is present in your table (e.g., 2).
Find out the total number of combinations in a table (e.g., 64).
Divide the number of your genotype by the total number of combinations (e.g., 2/64). To make it a percentage, multiply it by 100.
Genotype probability = Number of genotypes in the table/ total number of combinations * 100
Yay, you got it! 
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