Add parent category class to post_class() function

gistfile1.php

<?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 );
			$classes[] = 'category-parent-' . sanitize_html_class( $parent_cat->slug, $parent_cat->term_id );
		} else {
			$classes[] = 'category-parent-' . sanitize_html_class( $cat->slug, $cat->term_id );
		}
	}

	return $classes;
}
add_filter( 'post_class', 'mamaduka_add_parent_category_class', 10, 3 );