Carl Correns
The occurrence of cases in which the heterozygote is intermediate (the absence of dominance) was added in a footnote to his 1900 paper. That dominance was not always present had been seen and understood by Mendel, according to his letters to Nägeli. By a quirk of history Correns was a student of Nägeli, a renowned botanist with whom Mendel corresponded about his work with peas. Nägeli failed to understand how significant Mendel's work was.