Genetics and Farming

by E. R. SEARS

THE SCIENCE of genetics, which deals with the manner in which characteristics are passed on from generation to generation, is only about 50 years old, but already it has made many contributions to agriculture. The most spectacular undoubtedly is hybrid corn, whose value is even greater than the millions of dollars it has added to American farm income.

Hybrid corn is truly epochal in and of itself, but a greater good is the example, the impetus, and the key that it has given to all scientific breeding. It is at once a fruit and root of the discovery by the Austrian monk Gregor Mendel that characters are determined by unit factors, called genes, which remain unchanged in hybrids and which segregate out in succeeding generations to give new combinations of characters.

Because of Mendel's discovery, we can use nature's laws to produce more productive, better adapted, and healthier plants and animals. Many of our present-day crops and certain of our breeds of livestock owe some of their most desirable or even vital characteristics to the scientific breeder. For example, resistance to the deadly black stem rust was bred into our common wheats through a planned program involving crosses with related species.

Mendel made his basic studies almost a century ago (they remained unnoticed until the turn of the century), and the genetic experiments that led to hybrid corn were completed some 30 years ago. Since that time our knowledge of genetics has increased greatly, carried forward by man's intense interest in his own heredity and in that of other animals and of plants. Now, in 1947, we can confidently look ahead to a wide and profitable use by American agriculture of some of the more recent advances in genetic knowledge.

Hybrid corn owes its superiority in yield to hybrid vigor. When strains or varieties of corn are self-fertilized, or inbred, for a few years, the plants become progressively smaller and poorer yielding. In fact, many lines become so weak that they die out. Each surviving line becomes more and more uniform from year to year, however, with less and less further reduction in size and yield, and eventually becomes a constant, true-breeding type. When two such inbred lines, not closely related, are crossed with each other, the resulting “single-cross” hybridis usually larger and higher yielding than the original strains or varieties. This increase in size and yield is called hybrid vigor. Besides their higher productivity, hybrids of this sort are remarkably uniform, thus contributing to easier cultivation and harvest. Also, many undesirable characteristics, such as susceptibility to disease and lodging, may be eliminated in the process of inbreeding.

Thanks to genetic studies, we are now able to use hybrid vigor in other crops. We can do this because we have overcome the difficulties of crossing, or mating, plants that are normally self-fertilized.

In corn the female flowering organ, the ear, is separated from the male flowering organ, the tassel. Consequently, a plant may be self-fertilized or crossfertilizedat will, simply by shielding the ear until it is ready for pollination, and then applyingpollen from a tassel on the same plant (for selfing) or from another plant (for crossing).

In almost all of the other common crops, male and female organs do not occur separately, but are found together in the same flower. Each flower is "perfect," possessing both male and female parts, and self-fertilization normally occurs. In order to prevent self-fertilization, we remove the male organs from each individual immature flower--a process, called emasculation, too laborious for commercial use.

There are, however, some crops in which each plant, while possessing normal male and female organs, is infertile to its own pollen. The self-sterile plants include tobacco, rye, buckwheat, and certain fruits, vegetables, forage grasses, clovers, and flowers. In them cross-pollination is quite readily obtained, simply by growing the desired parental lines near one another. But in order to obtain the maximum vigor and uniformity in hybrids, one must have inbred lines for crossing each year—lines that have been self-fertilized for several generations. And in some self-sterile crops no satisfactory way has been found of selfing to obtain inbreds. In certain other self-sterile plants, pseudo self-fertility occurs, and these plants can be selfed successfully. In tobacco, for example, a plant may be self-sterile throughout the entire flowering season, except for the last few days. In this period of end-season fertility, self- fertilization occurs readily. Also, bud fertility occurs in various self-sterile plants, including tobacco and some members of the cabbage group, in which selfed seed may be obtained by opening an immature flower by hand and applying pollen from another flower of the same plant. By either of these methods the plant breeder can get inbred lines for use in production of commercial hybrid seed.