The way you write this seems to be implying that Mendel (a monk who lived hundreds of years ago and is responsible for our first understanding of genes by his experiments on pea plants) and Darwin who wrote Origins of Species, is somehow responsible for "medical discrimination due to high BMI" and eugenics?

Just because people use someone's work to justify something doesn't mean that the work itself was wrong or bad. Evolution isn't any less real because racists tried to imply certain types of humans were "less evolved" than others. Genetics isn't any less true because Nazis existed.