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Microbiology => Genetic Engineering => Breeding
Breeding
INTRODUCTION
Breeding, selective control of mating in plants and animals to produce organisms that better serve human needs for food, work, sport, or aesthetics. Simple breeding methods have been employed throughout human history. From paintings on the walls of Egyptian tombs, archaeologists have determined that dogs were bred at least 4000 years ago, and perhaps as early as 10,000 years ago. Ancient civilizations also domesticated varieties of cattle, sheep, goats, and grains.
The history of breeding can be divided into two periods: before and after the rediscovery in 1900 of Mendel's concepts of heredity. Before 1900, breeders worked primarily by selecting from each generation the animals or plants that displayed desired characteristics and then breeding these individuals to produce the next generation. Although this simple method, known as mass selection, produced some favorable results, it was often a slow and unpredictable process. After the rediscovery of Mendel's work, breeding became more predictable and scientific. Mendel's principles showed that many traits are transmitted as discrete units, and over successive generations these traits do not blend with or become corrupted by other traits. Most importantly, Mendel's work showed that by analyzing breeding results it is possible to predict which traits will occur, and in what proportions, in the next generation. Thus, analysis of breeding results gives the breeder knowledge about genotype (genetic makeup) of an organism based on observation of its phenotype (visual characteristics), and it provides a knowledge of genetic variation for specific traits. Genetic variation is the source for modification from which the breeder draws when selecting for a particular trait. Through these methods modern Mendelian breeding has produced remarkable improvements in a large variety of agriculturally useful organisms.
Animal and plant breeding has been responsible for vastly improving agricultural yields over the past several hundred years, and thus for improving the world's food supply. Systematic breeding programs emerged in Europe and the United States on an increasingly large scale during the Industrial Revolution in the late 18th century, partly in response to the demand for more food to feed an increasingly urban, nonagricultural workforce. In the 20th century, growing world populations have also brought increased pressure to improve agricultural yields.
ANIMAL BREEDING
Modern animal-breeding practices today are still based largely on mass selection, supplemented by three other methods: pedigree selection,family selection, and progeny selection. Pedigree selection focuses on the quality of the ancestors rather than of the individual. Pedigree selection is useful in evaluating young animals whose phenotypes are not fully developed, and in selecting for traits that are known to have high heritability. However, pedigree selection is a slow process. Family selection, based on analyzing the qualities of relatives, is faster. Family selection is often used in conjunction with individual selection, and it is valuable in estimating sex-limited traits (egg-laying ability or milk production, for example) in selecting the males from which to breed. Progeny selection involves selecting individuals based on the records of their progeny. Like family selection, it is useful when selecting for such sex-limited traits as milk yields in the progeny of a bull and traits with low or uncertain heritability. However, progeny selection is a slow process because it requires waiting for one generation or more to determine the quality of a given individual's offspring.
Since the mid-18th century, animal breeders have combined various methods of selection with inbreeding and outbreeding of stocks. Inbreeding involves crosses between closely related individuals. To fix or intensify a particular trait, herds or flocks are subdivided into smaller groups and intensively inbred for several generations. To increase vigor and avoid the accumulation of unwanted traits, individuals from these inbred stocks are then outbred, or crossed with members of other stocks. Outbreeding increases variability and produces new combinations of traits. Increasingly in the 20th century, as methods for freezing and storing sperm have been perfected, both inbreeding and outbreeding have been carried out by artificial insemination. Embryo selection is another method by which the breeder can increase desired traits in a population. In this method, fertility hormones are given to females that carry selected characteristics. Once the eggs have been fertilized, they are taken from the selected females and implanted in other females that carry them through the gestation period and then give birth. Embryo transfer is used less frequently than artificial insemination because it is more complicated and more expensive.
PLANT BREEDING
Plant breeding is similar in principle to animal breeding: Various forms of selection are combined with inbreeding and outbreeding. However, there are certain differences due to the unique aspects of plant reproduction. Unlike animals, many plants have the ability to self-fertilize or self-pollinate-that is, the pollen of one flower fertilizes the eggs within the same flower or the eggs of other flowers on the same plant. Other plants are cross-pollinators-the pollen from their flowers will not fertilize eggs from the same flower or plant. Due to persistent inbreeding, self-pollinating plants are genetically more uniform than cross-pollinated plants, which harbor more genetic variability. About half of the major agricultural plants-including wheat, rice, barley, beans, peas, and tomatoes-are self-fertilizing. The other half-including carrots, date palms, asparagus, hops, white clover, and cabbage-are cross-pollinating. Self-pollinating plants can reproduce without human intervention. Cross-pollination is more labor intensive because the breeder must prevent stray pollen from entering the flowers. To avoid stray pollination, the breeder must carefully remove the stamens (pollen-bearing organs) from a flower, dust the pistils (egg-producing organs) with the selected pollen, and then cover the flower with a small bag. The process of controlling pollination is somewhat easier in cross-pollinating forms such as maize (corn), in which the male and female organs are contained in separate flowers (the tassels bear the stamens, and the ear bears the pistils). In this case the breeder simply removes the tassels from those plants that are not to be used as pollinators. In modern breeding, the need for manual detasseling has been eliminated as a result of a genetic innovation known as cytoplasmic male sterility (CMS) in certain varieties of plants, including maize. CMS strains produce sterile male flowers, allowing breeders to place the plants in alternating rows of normal and CMS strains, thus ensuring maximum cross-pollination without high labor input.
Plant breeders supplement the methods of mass selection with other procedures, such as pure-line selection and hybridization. Pure-line selection involves continued inbreeding and selection for one or more desirable traits to produce a relatively homozygous (genetically homogenous) population. Hybridization involves crossing distinct strains or even species to produce hybrids (forms) that contain a combination of traits that is superior to that found in either parent. Because plants can form viable hybrids much more readily than animals, hybridization has long been a mainstay of plant breeding. The outstanding example of hybridization in the 20th century has been hybrid corn. Hybrid varieties have many superior qualities, especially in yield and vigor, but, because they do not breed true, farmers must buy new seeds each season.
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