Text box to appear after dropdown selection

I have the option to add additional text lines at an extra cost. At the moment a dropdown box has a yes or no to add text (the extra cost is then added at checkout) but even if no is selected the customer can still add the text in a text box. I would like the text box (for example, called line 2) only to appear after 'yes' has been chosen in the drop down box Is this possible?

Thanks.

Bob

Please indicate where on your site this is,

checkout,
products,
home page
contact us page......

Cheers

It is on the product page...

Thanks, will check it out.