The traits he picked- hope I get this detail pedantically correct: had only two alleles, each allele had an obvious phenotype controlled mostly by that gene, the two phenotypes were binary (no intermediate "half-wrinkled-half-smooth"). and all segregated independently (different chromosomes, or far enough that the linkage was extremely weak). I remember my high school teacher speculating he inspected many different phenotypes and then reported his results on the final ones he picked where the results were nice.

Unfortunately, most modern genotypes and phenotypes in humans don't follow these patterns, and over the years, genetics devloped an entire vocabulary and physical model to explain them, although at a fairly abstract level. None of it made any sense to me so I follow the biophysics/molecular biology approach which tends to consider many more underlying physical details

There's a related story, https://review.ucsc.edu/spring04/bio-debate.html and https://review.ucsc.edu/spring04/twoversions.html which shows how different fields think about the genotype/phenotype relationship. Grinding up steering wheels to figure out how they work...

Yeah that was my understanding. Most things have multiple genes controlling them. Like there is no single eye or hair color gene.