You can use data validation for that too!

select the range next to the range that has the yes/no dropdown.

let's say as in your first example, this range is D1:D27.

After selecting the range go to Data|Validation and choose Custom from the Allow menu. In the formula field enter: =C1="yes"

where C1 is the top most cell in the yes/no range.

Now if the user selects "no" in C1 and tries to type something in D2, he will get an error message.....

Note: this error message is editable to your taste via the other tabs in the Data Validation dialogue box.