Jones’s second contribution was theoretical and addressed the question of how heterosis worked: what was the mechanism behind hybrid vigor? His model combined the work of several different scholars. In 1910, Frederick Keeble and Caroline Pellew had published work showing the mendelian basis for hybrid vigor when two inbred strains of a plant — in their case, it was pea plants — were crossed. They were looking specifically at factors (we would call them genes) that made the hybrids taller than either of their inbred parents. What they discovered was that the tall hybrid combined two dominant genes, one from each parent. One gene, from on parent, was for a long stem, and the other, from the other parent, was for a thick stem. The result was that the hybrid was taller than either. This conclusion raised a question, however. Theoretically, it seemed that it should be possible to isolate all the desired genes in one plant through traditional breeding methods, by selecting plants with the desired characteristics and combining and recombining them, eliminating plants with undesirable combinations, until plants appeared that combined all the desired traits and bred true generation after generation. Yet this did not seem to be the case. There were some traits that seemed to be nearly impossible to separate and recombine. Why? To address this, Jones drew on the work of Columbia University geneticist Thomas Hunt Morgan, who had determined that sets of genetic traits were linked together in groups. The more closely two traits were linked, the more difficult it was to separate them — in some cases, it was virtually impossible. In today’s terms, Morgan had discovered that genes are grouped together on the chromosomes, and the closer together on the chromosome two genes lie, the less likely it is that they will separate during the genetic ‘shuffle’ that happens during reproduction. Jones realized that this type of linkage that Morgan had discovered was the reason why it was far easier to reach desired combinations of genes through inbreeding and hybridization than it was through traditional breeding practices. The upshot is that, as corn geneticist Paul Mangelsdorf put it in his 1975 biographical essay on Jones, that Jones’s “method of seed production made” growing hybrid corn “feasible, and his theory of heterosis made it plausible.” It was easier to convince agronomists to use the method once there was a convincing model for how it worked.
Photo Credit: W. Ralph Singleton papers, University of Virginia Special Collections.