In discussion I focused more on population genetics rather than inheritance of co-dominant alleles, since I think they have already learned and understand about the latter.
I think by this time the students know how to write their discussion, just they need more practice to 'bridge' the result obtained in class with the theory and making comparison with other data. They pretty much understand how to make conclusion. I also include err... may be pseudo experimental design, just want to give a little exposure on how scientific experiment is done. A little touch of statistics, and I encourage them to think how to modify the method if necessary.