Creating Alerts for Non-matches

I am continuing my quest to develop a timetable on Excel.......

Students choose different electives, upto a maximum of 20. In the example below, I have shown them with only four electives. Some choices require a pre-requisite subject to be chosen. In the example below, let us assume that you must select WXY101 in your choices if you want to include WXY102 as an option as well. Jane Doe fits this criteria, so there is no alert.

However, John Smith is trying to complete WXY102 without having chosen WXY101, and hence WXY102 appears in the alert column as a problem selection. Note that choosing WXY101 without choosing WXY102 is OK, as you can just complete a pre-requisite subject.


Obviously I have manually typed the subject code in the alert column - is there a way to automate this via a formula?

Any help would be greatly appreciated!
Cheers
Paul