I can relate to your frustration. I thought it was only about my school then, little did I know it is everywhere. I once conducted an experiment for a week and got nothing simply because the chemicals were 'suspected' to have expired, as said by the lab technologist. One week of my life gone just like that. Well, talking about homozygousity and heterozygousity, it goes beyond just donating alleles from the two parents but you have described it well for a layman to understand. Sometimes, recombinations and several other genetic processes take place before a character is manifested.
Of course it's a bit more complicated than I described, the whole post could have been 5k words longer had I properly explained everything :D for now, I think the explanation serves its purpose.