If your theme is based on the Genesis theme, instead of using the Page template dropdown option on the Index create/edit page to select a page template, you should use the theme's template selection box. 

Use the Genesis layout settings box to select a page template

This will normally be displayed underneath the Index options but if you don't see it, click on the Screen Options tab at the top right hand side of the page and make sure that Layout Settings is checked.

If you don't see the Layout Settings selection box on the page, make sure the Layout Settings option is checked in Screen Options