Combo Box

1.On click on the downward arrow it should be expand.
2.Combo box should contain the options Yes/No
3.After selecting the Yes or No box should be shorten and 
contain the selected option. 

Default Status - Combo Box should be blank

Click on the combo Box - It should display the 1. Yes     
2. No items in the List.

Select Yes / No - Selected item should display in the Combo 
Box.

Try to type something in the combo box - It should not 
accept any value in the combo box.

Apart from the above mentioned test cases you can also check
for 'consistency' in the under-lying layer like boolean
variable  , db column etc to capture the UI element in the
database.