Regarding DropDown in Grid of C#.Net

Question

I have a DropDown in GridView.I populated that DropDown manually via Items and putting those Items in the "Collection".The Items are :Mr and Ms.Its working well and getting populated.
My problem is, while I press the down arrow key from that Cel, the Items get changed i.e,Mr changes to Ms and vice versa despite my requirement. My requirement is that, while we press down arrow Key,the control should Go to Next line of that GridView instead of Items getting changed.
Please Suggest me on this Issue.
Thank you.