Heterosis in corn

​As it became clear that corn hybrids could create large boosts in yield when two unrelated inbreds were crossed, it became clear that the biggest gain came when the inbreds were derived from distinct heterotic groups.  Lancaster Sure Crop was created in Lancaster county Pennsylvania.  It featured long, flinty ears with disease resistance consistent with the humid eastern USA environment.  Meanwhile, the Reid dent corn, developed from the accidental crosses of New England Flint with Southern dent in central Illinois spread throughout the corn belt with many sub-varieties selected for differing moisture and soil stresses.  These two heterotic groups became major sources of inbreeding in the 1920 and 30’s as hybrid development advantages were clear.
 
In 1935, George Sprague began intercrossing 16 inbreds and 4 inbred parents that were mostly from these many sub-varieties of Reid dent corn.  This became known as Iowa Stiff Stalk Synthetic population (SS).  New inbreds from this populations were test crossed with Lancaster inbreds, the successful SS inbreds were intercrossed, new selfs made, crossed again to Lancaster inbreds.  These cycles were continually resulting in more populations at Iowa State University.  Other public and private corn breeding efforts built on Stiff Stalk Synthetic, selecting from portions of it as well as constructing their own genetic populations.  Inbreds from these efforts continuously improved hybrid performance as national corn yields increased. A third heterotic group, Iodent, was developed by mostly private breeders in the USA.  Internationally, similar processes of identifying inbreds that combined with unrelated inbreds to give hybrid vigor.
 
The cause of heterosis lack some clarity but most of the evidence implies that negative, recessive genes in one inbred are overcome by the dominant version of the gene in the other inbred.  Given that a corn plant has at least 30,000 genes, and that any selfing, whether intentional to make inbreds or coincidental in open pollinated varieties, probability of homozygous recessive genes is high. Despite some randomness in matching dominant genes for the recessives of the other parent, inbreds derived from heterotic populations increase the probability of covering up the negative recessive genes.