What do su, se, sh mean after sweet corn names?

There are three types of genes and sweet corn. It is the genes that cause corn to form sugar instead of starch in the kernels. The first sweet corns discovered are called normal, or sugary-1 types. These varieties are designated su-1 in catalogues. Their kernels have starch and sugar in them. The second type of sweet corn discovered has no starch in the kernels, only sugar. They are called supersweet, or shrunken 2 (sh-2) corns since the seeds a shriveled when dry. These are the sweetest corns. The third type is called sugary enhanced (se) corn. They have more sugar than su-1 types, but do have starch in the kernels.