Question

What was Mendel's explanation for the fact that 0% of the offspring from this cross were green?

Answer 1

So Mendel got roughly 75% yellow and 25% green in f2. This means that f1 contains all heterozygous genes, as shown below

Y y
Y YY Yy
y Yy yy
Summary: 75% YY or Yy, 25% yy

Which means that green (y) is a recessive phenotype, while yellow (Y) is a dominant phenotype, since plants with heterozygous genotype Yy express the yellow trait.