1. Long-inbred lines of corn infrequently show heritable variations. A search among all the inbred material available over a period of several years has revealed deviating lines that differ from the original type in some distinct morphological or physiological character. Presumably these variations are single point mutations, although it is difficult to separate primary changes from delayed segregations. All variations so far found appear to be degenerative changes, reducing the ability of the plant to grow and to reproduce itself. They include delayed flowering, leaf blotching, narrow leaf, reduced plant size at maturity, crooked stalk and chlorophyll alterations.
All of these have occurred naturally. In X-rayed material less conspicuous variations have been found but these are not sufficiently well marked to segregate clearly.
Four of the natural variations have been crossed back with the normal lines from which they come. All have given the surprising result of a hybrid-vigor effect. The F1 plants are either taller, greener, broader in leaf and stalk, earlier in flowering or more productive of grain. The differences are small but measurable. If it is proved that these differences involve only a single gene this would be clear evidence that heterosis is something more than an accumulation of non-allelic dominant favorable growth factors.
It may also be questioned fairly whether these are actually the degenerate types that they seem. From evidence previously reported these reduced lines may give superior results in outcrosses. Since those mutations presumably originate in the heterozygous condition, the plants containing them should be more vigorous than the homozygous individuals in the same line and are likely to be selected for propagation. This was actually the case in the blotched leaf line that came originally from a plant selected as superior in height of stalk and ear development to the other plants in the same self-fertilized progeny. This is additional evidence to show why inbred lines, are difficult to maintain in a constant and uniform condition.
It may also explain why some of the poorest lines are so useful in production of commercial hybrids. For example, Iowa L317, C.I. 540 and 4-8 are notably unsatisfactory as inbreds but are used in hybrids that are widely grown. Combining ability results from a complementary action that is not clearly indicated in the homozygous condition and apparently involves an equilibrium of genic material that is not as yet fully understood.