What file is the tag located in Genesis default theme?

In every good theme, the </body> tag is located in the footer.php file.

However, unless you’re using a child theme, any modification will be lost on a theme update. That’s why it’s better to use the wp_footer filter hook like so:

add_action( 'wp_footer', 'wpse_76330_aweber_tracking_code' );

function wpse_76330_aweber_tracking_code() { ?>

    %%%% insert tracking code here %%%%

<?php }