Goosegirl:
In the mid-1900’s, some very important new sweet corn genes were developed: ‘SH2’ shrunken gene and the ‘SE’ sugary enhanced gene.
The SH2 gene greatly boosts the amount of sugar in the endosperm (the part of the kernel where energy is stored for later use by the developing embryo), and that’s why SH2 varieties are often called ‘supersweets’. The seeds of these varieties have less starch in them, so they appear shriveled and weigh less than other sweet corn seeds. (Because of the shriveled characteristic of the seed, they are sometimes referred to as shrunken 2 or just SH2.) They can be a challenge to grow, especially in the north, because they need warmer soil for good germination. The SH2 gene slows the conversion of simple sugars to starch, so supersweet varieties have a much wider harvest window than other genotypes. SH2 varieties must be isolated or they will become tough and starchy, and they will also ruin SU or SE varieties grown next to them, if the pollen happens to cross.
The SE gene gives varieties improved eating quality over SU varieties by slightly increasing the level and changing the types of sugars in the kernels. SE varieties also have a very tender pericarp (the outer layer of the kernel). The SE gene does not slow the conversion of sugars to starch but the harvest window is slightly longer than with SU varieties because of elevated sugar levels. SE varieties do not require isolation from SU varieties, but they do require isolation from SH2 and field or popcorn.
Some SE varieties are sweeter than others, depending on whether one or both of their parents were sugary enhanced. Varieties that get the SE gene from both their parents are homozygous for that trait, or ‘double SE,’ and all of their kernels have the SE characteristics. Varieties that have just one SE parent are heterozygous, or ‘single SE,’ develop 25% of their kernels with SE traits. Typically a homozygous SE will have better eating quality than a heterozygous SE. For example, Tablesweet™ varieties such as Parfait are homozygous SE with high sugar levels and slightly better holding ability.
Within the SU and SE genotypes, modifier genes are responsible for differences in flavor. A variety can be homozygous or heterozygous for a modifier gene, just like the SE gene. Modifier genes are what allow breeders to develop so many different varieties. They are why it’s possible to have an 80-day, bi-color homozygous SE corn that has better eating quality that another variety that shares those same features.
In recent years new varieties have been developed that have different combinations of the three major genes (and their modifier genes) ‘stacked’ together. The goal of breeders is to put together the best characteristics from SU, SE and SH2 genes.
‘Synergistic’ varieties are heterozygous or homozygous SE, with the addition of some SH2 kernels. Sweet Breeds™ varieties such as Sweet Chorus and Sweet Rhythm have about 25% SH2 kernels, 25% SE kernels and 50% SU kernels. The seed has SU vigor but higher sugars. Triplesweet™ varieties such as Providence have 100% SE kernels, 25% of which also have the SH2 trait. These are like double SE varieties in vigor, but sweeter.
‘Augmented SH2’ varieties are supersweet types that also have the SE trait in all their kernels. Examples include Gourmet Sweet™, Multisweet™ and Xtra-Tender Brand™. These are high in sugars, slow to convert to starch, and tender like an SE. Mirai has all three genotype traits in its kernels: SH2, SE and SU.
Hope that helps
Cheers,
Balders
http://www.uvm.edu/vtvegandberry/factsheets/corngenotypes.htmlattribution added for copyright 
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May 10, 2010, 11:02