Validate Cascading DropDownList (One dependent on another) using JavaScript

Needing help to validate multiple dropdown lists. I am needing to figure out how I can create a C# code that allows this function.

Ex:

DropDownList1 select 'option1'  then DropDownList2, DropDownList4, DropDownList7 will be required to be filled out.

Using Button click will make a pop up alert window to tell users that they need to fill out a certain dropdownlist if that dropdown list is blank.