Genetics : Improving Plants And Animals (1963)

The sun. [Music] The oldest civilizations we know cultivated plants for food and raise domestic animals similar to varieties that we raise today. Yet the animals we raise today are superior in many respects. Through the science of genetics, they have been improved so that they provide more meat with better flavor. They have also been improved so that they are more resistant to disease. The many plants we raise today have also been improved. For example, these Macintosh apples have been improved so that their size has been increased. Some of the plants we have today, such as grapefruit, were not developed until modern times. Man's ability to make changes in plant and animal stocks has resulted in benefits to all of us, including an increase in our real wealth and improvements in our health. The development of better plant and animal stocks is one application of the science of genetics. Geneticists know that the characteristics of all living things such as these fruit flies are determined by the genes found in the chromosomes of all cells. In this photo microraph, we see the chromosomes in the cells of a salivary gland of a fly. It is believed that the dark bands in these chromosomes contain the genes which are transmitted from one generation to the next through the reproductive cells. Because of their knowledge of how characteristics are inherited, geneticists are able to change characteristics of living things. For example, the small brown jungle fowl is the ancestor of all chickens. By changing the characteristics of this fowl, scientists have produced many varieties of domestic chickens for better egg production or meat production. Sometimes plant and animal stocks are altered for aesthetic reasons. This dog is a result of scientific breeding to produce a pleasing appearance and form. These tulips are an example of plants developed for beauty. So improving plant and animal stocks means changing them to produce an organism with the desired inherited characteristics. To do this, four different techniques may be used. One technique is known as inbreeding. Another method is hybridization or crossbreeding. A third method is selecting and developing desired mutations. Another way is to increase the number of chromosomes in the reproducing cell. Let's see how animal characteristics can be improved through inbreeding. Inbreeding is sometimes called pure line breeding. For example, this inbred herd of dairy cattle from common parents has a high rate of milk production. But several of these related cows have exceptionally high rates of milk production. By selecting only those cows that exhibit the characteristic of high milk production and inbreeding them to a bull like this one that comes from a related line of milk producers, the breeder will continue to improve the milk production of his herd. So inbreeding does increase the chance that the desired characteristic will appear in the offspring. However, inbreeding sometimes results in the development of undesirable characteristics. For example, this cow does have the characteristics of high milk production. But if it has also developed an undesirable characteristic such as low resistance to disease, the breeder can use a second method to further improve his animal stocks. This second method is hybridization or crossbreeding. We'll start with this Brahman bull. The Brahman is well suited to living in hot climates, but Brahman cattle are not good meat producers. Why not combine the genes from this animal that determine the characteristic of heat resistance with the genes from an animal that has the characteristic of good meat production? Short horn cattle, for example, are excellent meat producers. But shortorth horns do not thrive in a hot climate. However, through hybridization by crossbreeding the short horn and the brahman, a hybrid is produced that has the desired characteristics of both parents. good meat production and resistance to a hot climate. This new breed, the Santaertrudis, is one of the notable examples of hybridization. Developed after careful selective breeding through several generations, the Santaertrudis today is an established breed ideally suited to the hot climate of the southwestern United States and similar rangeands. The desired characteristics are inherited by succeeding generations. Also, hybrid offspring of this kind often grow more rapidly and mature into larger specimens than either parent. This is called hybrid vigor. Another outstanding example of hybridization was achieved in 1941 with turkeys. Specimens of the large domestic turkey were developed through generations of selective breeding to a smaller, fleshier bird called the beltsville turkey. Corn can also be improved by hybridizing to produce desired characteristics of size, fullness of ear, and yield per acre. The many diseases that afflict corn may also be overcome by producing hybrids that are resistant to certain diseases and other afflictions. For example, these corn plants are leaning, a condition described as poor standability. To produce better standability, such plants may be crossbred with a type of dwarf corn that has a shorter, much sturdier stock. This desired characteristic can be transmitted to the hybrid offspring which is both tall and sturdy. In addition to the specific characteristics of a particular hybrid, hybrids often show the general characteristic of hybrid vigor, healthy, productive plants that are an improved source of food. Now let's see how an accidental change can be used to improve plant and animal stocks. For example, cattle such as these herfords normally have horns, but sometimes a calf is born that does not have the horns found in both parents. Such an animal is called a mutant. This herford calf is a mutant. There are no signs of developing horns on the animal. The breeder knows that this mutation is the result of an accidental change in a gene. Since hornless cattle are easier to handle, the breeder wants to retain this characteristic. When mature, this animal will be bred with normal cattle that do have horns. The gene that causes the hornless characteristic will be transmitted to at least half of the offspring. Then the mutant offspring of succeeding generations will be inbred to produce a new line of cattle. They are called hornless or pold cattle. Mutations may also occur in plants. Fruits, for example, normally have seeds. The grapes at the left have seeds. These grapes are called seedless. that is they have only tiny rudimentary seeds. The seedless grapes are mutants. All the grapes on this particular vine are seedless. Seedless grapes are desirable because they are easier to eat. To propagate this mutant, plantreeders usually take a stem from the vine when it is dormant. The stem or cutting of the mutant grape can be planted in soil. The young grapeshoot develops into a vine with leaves and roots through vegetative reproduction. When this vine is full grown, it still has the same genes which produce the seedless grapes and so it will continue to bear seedless grapes. Sometimes plant mutants result in another kind of improvement. Perhaps you've seen unusually large fruit such as this Macintosh apple. The mutant apple is nearly twice the size of normal Macintosh apples. The increase in size is a result of an increase in the number of chromosomes in each cell of the larger apple. In this drawing, we see the normal number of 34 chromosomes found in each cell of the smaller apple. Here we see the chromosomes from the larger apple. There are 68 chromosomes, double the amount in each cell in the larger apple. Today, plantreeders are experimenting with certain chemicals such as colultesine, which artificially increases the number of chromosomes in each plant cell. Here, colchesine is being placed on a young grape vine. By increasing the number of chromosomes, breeders are artificially producing larger plants and larger fruit. Here are normalsiz grapes compared with large grapes. The doubling of the chromosomes has produced the larger fruit. Certain flowers too such as African violets have been increased in size by using cultine. Another result of a change in genes is doubling in flowers. These are normal tulips with single flowers. These mutants are double tulips. Desirable characteristics in plants include not only size or beauty but resistance to disease. These sugar beets are only one of many examples of improvement of food crops achieved through plant breeding. These beets have been improved so that their leaves are resistant to a disease called leaf spot. Notice the healthy leaves compared with this diseased leaf. So plants and animals are improved to achieve a number of different desirable characteristics. certain size, tastier, more tender meat, better flavor, lack of seeds, larger fruits, and other characteristics. Such improvements are achieved by using four different techniques to change inherited characteristics. We improve both plant and animal stocks through inbreeding, by hybridization, by developing desirable mutants, and by increasing the number of chromosomes. These are the four principal techniques used to control inherited characteristics. [Music] As geneticists learn more about how characteristics are inherited, they will be able to achieve even more remarkable results in improving our plants and animals. [Music]