I'm interested in your reasoning. I teach Mendelian genetics because 1) it dovetails with segregation & assortment of chromosomes; 2) teaches importance of math & stats; 3) relevant to many disorders with relatively simple inheritance; 4) invites thinking; 5) accessible to novices.

If you do all these in an undergraduate genetics course, it is certainly part of the foundations. But when school students are only taught about alleles controlling traits and Punnet squares that show a direct correspondence between alleles and phenotypes, then we have a problem.