The Advent of Genetics and the Science of Hybrid Corn

Other scientists, impatient with the ear-to-row method, gravitated to the new science of genetics. In 1900 three European botanists independently rediscovered the paper Austrian monk Gregor Mendel had published in 1866 on pea hybridization. Mendel had shown that particles (genes) code for traits in pea plants, and after 1900 scientists extended his insight to other plants and animals. American geneticist George Harrison Shull, in Cold Spring Harbor, New York, chose to work with corn because it differed from peas in that it cross-fertilized rather than self-fertilized, and Shull wished to know whether Mendel’s laws of inheritance held for crossfertilizing as well as inbreeding populations. The crucial insight came in 1909 when Shull realized that inbreeding would reduce corn to several uniform populations, with each population similar to a variety of peas in its uniformity of genes. A breeder could then hybridize corn as Mendel had hybridized peas.

Hybridization held out the possibility of imparting heterosis, or hybrid vigor, to corn plants. For centuries humans had known that hybrids―the mule, for example―have greater vigor than their parents. In corn, hybrid vigor might display itself as high yield, resistance to diseases, insects and drought, stalk strength, or some combination of desirable traits.

Although simple in conception, the practice of breeding corn for hybrid vigor challenged a generation of agronomists. The problem lies in the biology of corn, which, as we have seen, cross-fertilizes rather than self-fertilizes. The tassel, which contains the pollen, and the silk, which holds the ovule, are far apart in corn, and wind carries the pollen from one plant to the silk of another. To inbreed corn, an agronomist or farmer or anyone who knows the anatomy of a corn plant must cover the tassel and silk of a plant to prevent pollen from one plant from blowing onto the silk of another. When the tassel is mature, the breeder collects the pollen and spreads it on the silk of the same plant. Several generations of inbreeding produce homozygous lines that breed true, as a variety of peas does. In turning the natural process of crossbreeding on its head, however, inbreeding weakens corn, producing scrawny ears with fewer seeds than the parents. This paucity gives the breeder enough seed to make a cross on a tiny plot but too little to make a cross on the scale that farmers need for their fields. As long as the seed yield was small, hybrid corn remained a curiosity rather than a commercial venture. But in 1917 agronomist Donald F. Jones at the Connecticut Agricultural Experiment Station got past the seed bottleneck by crossing four inbred lines over two generations. Jones crossed the four inbreds in two groups of two to yield two hybrids, and then crossed these two hybrids to produce one second-generation hybrid. Because this method required two generations of breeding, Jones called this hybrid a double cross to distinguish it from the single cross, a hybrid produced by one cross of two inbreds over one generation.