If a true-breeding twelve inch plant (this means when self-crossed the offspring are always 12 inches) is crossed with a true-breeding six inch plant and all of the f1 plants are nine inches tall, this is most likely a situation of?

This result suggests the plant heights are not fully explained by classic Mendelian inheritance. In Mendel's work he argued that one of the genes from one parent would be dominant. In this case, rather than having some plants resemble the parent with a "dominant" gene there is a blending of the two height genes. This is a form of incomplete dominance which is a type of Non-Mendelian inheritance.