Mamaduka

<?php
/**
 * Add parent category class to post_class() function
 *
 * @link http://wordpress.stackexchange.com/q/23259#23260
 */
function mamaduka_add_parent_category_class( $classes, $class, $post_id ) {
	foreach ( (array) get_the_category( $post_id ) as $cat ) {
		if ( ! empty( $cat->parent ) ) {
			$parent_cat = &get_category( $cat->parent );