What do you mean by a two chromosome species? A quick google says pea plants have 14 chromosomes. I only looked because I had never heard of a species only having two chromosomes. Do you mean the traits he was selecting for only had two alleles?

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...

Diploid. Peas have 7 pairs of chromosomes. Many types of plants have multiple (polyploid) copies of each chromosome, and you will not discover the classic Mendelian dominant-recessive pattern by studying them. Mendel lucked out by studying a diploid species. https://duckduckgo.com/?q=polyploid&ia=web

Right, my bad for wrong terminology as biology nor botany is not my speciality either. I was thinking along the lines of XY for humans, then the ordinary two row, two column chart used to teach the basics of Mendel with pea plants and dominant/recessive non polygenic traits in introductory biology classes.