Probability / Statistics (Maybe Venn Diagram/Boolean)

#1
So this is a bit confusing and I will try to explain the best way possible:

1 parent 1 parent = 2 Parents

+ + = No
+ - = No
- + = No
- - = Yes

So you have a 1/4th (25%) chance of yes in 2 parents.

What is the chance of yes per parent?