I've used the Genetics with a Smile lesson in every class I've taught since my internship.  In middle school, we used it to review the differences between sexual and asexual reproduction, and last semester I used it to teach dominant and recessive traits and as a general illustration of genetics.  This semester I tied it into our lesson on meiosis (Mitosis & Meiosis: Doing it on the Table) by having the students first write the diploid genotype of their parental smiley, then segregate the chromosomes into gametes, and finally combine gametes with a partner to form a zygote, whose genes they translated into an illustrated phenotype.  The kids love to color for a change, and I usually post the smileys around the room as a visual cue.

The version of the lesson I use is from Columbus City Schools, modified from T. Trimpe's lesson from Science Spot.  During my overachieving intern days, I actually made 25 sets of 11 diploid paper chromosomes for use with this activity, and I am sooooooooooooo glad I kept them because I've used them multiple times.  (This is probably why teachers are such packrats, because everything is either expensive to buy, time-consuming to make, or both!)

Awkward Family Photo