drop down list with if yes please specify option

How do I allow the user to specify what data they want backed up if they select yes in the drop down list below.

Data Recovery Required?: <select name="dataRecYN" id="DataRec">
 <option>Please Select</option>
 <option value="J_DataRecY">Yes</option>
 <option value="J_DataRecN">No</option>
</select>

Thanks