The only genetic difference between sweet corn and field corn is that in sweet corn one gene for the metabolism is modificated (in this case recessive).
When the reserve material is deposited in the nutritive tissue, sugars are produced in the first step, which are then transformed into starch. The sweet corn or sugary gene (su1) slows down this process, i. e. the sugar production continues, the content of sugar increases as a consequence of the slow transformation into starch, and thus the milky kernel has a distinctly sweet flavour.
When sweet corn is intercrossed with field corn, the “correct” gene in standard corn suppresses this characteristic completely, even if, as explained above, the sugar gene su1 exists in duplicate form. Geneticists call this a dominant hereditary process.