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breeding, in agriculture and animal husbandry, propagation of plants and animals by sexual reproduction; usually based on selection of parents with desirable traits to produce improved progeny. In conventional breeding, progeny inherit genes for both desirable and undesirable traits from both parents. Breeders conserve desired characteristics and suppress undesirable ones by repeatedly selecting meritorious individuals from each generation to be the parents of the next. This process leads to a population expressing a combination of inherited traits that distinguishes it from the rest of the species. In plants, such a population is described as a variety or cultivar; in livestock, it is called a breed. Purebreds result from one or more generations of inbreeding, or mating of close relatives, such as brother to sister or offspring to parent (backcrossing).
Inbreeding produces families or lines with increasing degrees of genetic uniformity, or homozygosity, in successive generations. In highly homozygous families, dominant genes are uniformly transmitted and expressed; recessive genes are also more likely to be expressed, and to produce undesirable traits, including loss of general vigor and fertility. In some plants, such as wheat, that are naturally self-fertilizing and homozygous, deleterious traits are readily eliminated by natural selection; there is no loss of vigor.
In naturally cross-pollinated or open-pollinated plants, and in animals, loss of vigor in inbred lines can be restored by outbreeding to unrelated or distantly related lines; a first-generation hybrid is more vigorous than either of its purebred parents. Animal breeders exploit the phenomenon of hybrid vigor, or heterosis, in producing crossbred cattle, sheep, swine, and other domestic animals. Much of the corn (Zea mays mays) grown in the United States and other agriculturally developed countries is the hybrid of two different inbred lines, or the double-cross hybrid of four inbred lines.
Selective breeding developed with the domestication of useful species during the Neolithic period: the oldest known remains of cultivated crops and domestic animals show signs of purposeful improvement. For centuries, selective breeding proceeded empirically. Beginning in the 18th cent. various breed associations formed to register purebred herds and flocks and keep track of pedigrees. Plant breeders collected seeds and documented their genealogies. The basic principles of heredity, originally published by the Austrian biologist Gregor Mendel (see Mendel, Gregor Johann) in 1866, were rediscovered in 1900.
With subsequent discoveries in genetics, and progress in artificial insemination and other breeding techniques, plant and animal breeding have become increasingly scientific. More recent advances in biotechnology and genetic engineering allow breeders to transfer specific genes and gene complexes among plants and animals, bypassing the limitations of conventional sexual reproduction. Knowledge of genomes and the techniques of genetics also enhance conventional breeding: In marker-assisted breeding, genetic markers are used to identify the desired characteristics in a plant while it is a seed or seedling, reducing the time needed to find the most promising individuals with those traits. Seeds and seedlings selected using marker-assisted breeding must still be grown and evaluated and then subjected to field trials in a variety of growing regions to determine their ultimate value.
The application of genetic principles to improving heredity for economically important traits in domestic animals. Examples are improvement of milk production in dairy cattle, meatiness in pigs, feed requirements or growth rate in beef cattle, and egg production in chickens. Selection permits the best parents to leave more offspring in the next generation than do poor parents.
Selection is the primary tool for generating directed genetic changes in animals. It may be concentrated on one characteristic, may be directed independently on several traits, or may be conducted on an index or total score which includes information on several traits. In general, the third method is preferable when several important heritable traits need attention. In practice, selection is likely to be a mixture of the second and third methods.
Heritability, the fraction of the total variation in a trait that is due to additive genetic differences, is a key parameter in making decisions in selection. Most traits are strongly to moderately influenced by environmental or managemental differences. Therefore, managing animals to equalize environmental influences on them, or statistically adjusting for environmental differences among animals, is necessary to accurately choose those with the best inheritance for various traits.
The improvement achieved by selection is directly related to the accuracy with which the breeding values of the subjects can be recognized. Accuracy, in turn, depends upon the heritabilities of the traits and upon whether they can be measured directly upon the subjects for selection (mass selection), upon their parents (pedigree selection), upon their brothers and sisters (family selection), or upon their progeny (progeny testing). For traits of medium heritability, the following sources of information are about equally accurate for predicting breeding values of subjects: (1) one record measured on the subject; (2) one record on each ancestor for three previous generations; (3) one record each on five brothers or sisters where there is no environmental correlation between family members; and (4) one record each on five progeny having no environmental correlations, each from a different mate.
Propagation of improved animal stocks is achieved primarily with purebred strains descended from imported or locally developed groups or breeds of animals which have been selected and interbred for a long enough period to be reasonably uniform for certain trademark characteristics, such as coat color. Because the number of breeding animals is finite and because breeders tend to prefer certain bloodlines and sires, some inbreeding occurs within the pure breeds, but this has not limited productivity in most of these breeds. Crossbreeding makes use of the genetic phenomenon of heterosis. Heterosis is improved performance of crossbred progeny, exceeding that of the average performance of their parents. Most commercial pigs, sheep, and beef cattle are produced by crossbreeding. See Genetics
Advances in a variety of technologies have application for improvement of domestic animals, including quantitative genetics, reproductive physiology, and molecular genetics. Quantitative geneticists use statistical and genetic information to improve domestic animals. Typically a statistical procedure is used to rank animals based on their estimated breeding values for traits of economic importance. The statistical procedures used allow ranking animals across herds or flocks, provided the animals in different herds or flocks have relatives in common. The primary contribution of reproductive physiology to genetic improvement is to reduce the generation interval. If genetic improvement is increasing at the same rate per generation, more generations can be produced for a fixed time, and thus more gain per unit of time. The most important development was artificial insemination, which allows extensive use of superior males. Another development was embryo transplantation, which allows more extensive use of females. Cloning is a relatively new technique, by which whole and healthy animals have been produced that have the same DNA as the animal from which the cells were taken.
Due to advances in molecular genetics, knowledge is increasing regarding the location of genes on chromosomes and the distance between the genes. In domestic animals, polymorphisms (changes in the order of the four bases) that are discovered in the DNA may be associated with economic traits. When the polymorphisms are associated with or code for economic traits, they are called quantitative trait loci (QTL). When a few or several quantitative trait loci are known that control a portion of the variability in a trait, increasing the frequencies of favorable alleles can enhance the accuracy of selection and augment production. Another use of molecular genetics is to detect the genes that code for genetically predetermined diseases. An example is the bovine leukocyte deficiency gene, which does not allow white blood cells to migrate out of the blood supply into the tissues to fight infection. The calves perish at a young age. Screening all sires that enter artificial breeding organizations and not using sires that transmit the defect has effectively controlled this condition.
The application of genetic principles to improve cultivated plants. New varieties of cultivated plants can result only from genetic reorganization that gives rise to improvements over the existing varieties in particular characteristics or in combinations of characteristics. Thus, plant breeding can be regarded as a branch of applied genetics, but it also makes use of the knowledge and techniques of many aspects of plant science, especially physiology and pathology. Related disciplines, like biochemistry and entomology, are also important, and the application of mathematical statistics in the design and analysis of experiments is essential. See Genetics
The cornerstone of all plant breeding is selection, or the picking out of plants with the best combinations of agricultural and quality characteristics from populations of plants with a variety of genetic constitutions. Seeds from the selected plants are used to produce the next generation, from which a further cycle of selection may be carried out if there are still differences. Conventional breeding is divided into three categories on the basis of ways in which the species are propagated. First come the species that set seeds by self-pollination; that is, fertilization usually follows the germination of pollen on the stigmas of the same plant on which it was produced. The second category of species sets seeds by cross-pollination; that is, fertilization usually follows the germination of pollen on the stigmas of different plants from those on which it was produced. The third category comprises the species that are asexually propagated; that is, the commercial crop results from planting vegetative parts or by grafting. The procedures used in breeding differ according to the pattern of propagation of the species. Several innovative techniques have been explored to enhance the scope, speed, and efficiency of producing new, superior cultivars. Advances have been made in extending conventional sexual crossing procedures by laboratory culture of plant organs and tissues and by somatic hybridization through protoplast fusion.
The essential attribute of self-pollinating crop species, such as wheat, barley, oats, and many edible legumes, is that, once they are genetically pure, varieties can be maintained without change for many generations. When improvement of an existing variety is desired, it is necessary to produce genetic variation among which selection can be practiced. This is achieved by artificially hybridizing between parental varieties that may contrast with each other in possessing different desirable attributes. This system is known as pedigree breeding, and it is the method most commonly employed, and can be varied in several ways.
Another form of breeding often employed with self-pollinating species involves backcrossing. This is used when an existing variety is broadly satisfactory but lacks one useful and simply inherited trait that is to be found in some other variety. Hybrids are made between the two varieties, and the first hybrid generation is crossed, or backcrossed, with the broadly satisfactory variety which is known as the recurrent parent. Backcrossing has been exceedingly useful in practice and has been extensively employed in adding resistance to diseases, such as rust, smut, or mildew, to established and acceptable varieties of oats, wheat, and barley.
Natural populations of cross-pollinating species are characterized by extreme genetic diversity. No seed parent is true-breeding, first because it was itself derived from a fertilization in which genetically different parents participated, and second because of the genetic diversity of the pollen it will have received. In dealing with cultivated plants with this breeding structure, the essential concern in seed production is to employ systems in which hybrid vigor is exploited, the range of variation in the crop is diminished, and only parents likely to give rise to superior offspring are retained.
Plant breeders have made use either of inbreeding followed by hybridization or of some form of recurrent selection. During inbreeding programs normally cross-pollinated species, such as corn, are compelled to self-pollinate by artificial means. Inbreeding is continued for a number of generations until genetically pure, true-breeding, and uniform inbred lines are produced. During the production of the inbred lines, rigorous selection is practiced for general vigor and yield and disease resistance, as well as for other important characteristics. To estimate the value of inbred lines as the parents of hybrids, it is necessary to make tests of their combining ability. The test that is used depends upon the crop and on the ease with which controlled cross-pollination can be effected.
Breeding procedures designated as recurrent selection are coming into limited use with open-pollinated species. In theory, this method visualizes a controlled approach to homozygosity, with selection and evaluation in each cycle to permit the desired stepwise changes in gene frequency. Experimental evaluation of the procedure indicates that it has real possibilities. Four types of recurrent selection have been suggested: on the basis of phenotype, for general combining ability, for specific combining ability, and reciprocal selection. The methods are similar in the procedures involved, but vary in the type of tester parent chosen, and therefore in the efficiency with which different types of gene action (additive and nonadditive) are measured.
Varieties of asexually propagated crops consist of large assemblages of genetically identical plants, and there are only two ways of introducing new and improved varieties: by sexual reproduction and by the isolation of somatic mutations. (A very few asexually propagated crop species are sexually sterile, like the banana, but the majority have some sexual fertility.) The latter method has often been used successfully with decorative plants, such as chrysanthemum, and new forms of potato have occasionally arisen in this way. When sexual reproduction is used, hybrids are produced on a large scale between existing varieties; the small number that have useful arrays of characters are propagated vegetatively until sufficient numbers can be planted to allow agronomic evaluation.
Cell technologies have been used to extend the range and efficiency of asexual plant propagation. For example, plant cell culture involves the regeneration of entire mature plants from single cells or tissues excised from a source plant and cultured in a nutrient medium. In micropropagation and cloning, tissues are excised from root, stem, petiole, or seedling and induced to regenerate plantlets. All regenerants from tissues of one source plant constitute a clone. Microspore or anther culture is the generation of plants from individual cells with but one set of chromosomes, haploid cells, as occurs in the development of pollen. Microspores are isolated from anthers and cultured on nutrient media, or entire anthers are cultured in this manner. Doubling of chromosomes that may occur spontaneously or can be induced by treatment with colchicine leads to the formation of homozygous dihaploid plants. See Plant propagation; Pollen
Breeding for new, improved varieties of crop plants is most often based on cross-pollination and hybrid production. Such breeding is limited to compatible plants, and compatibility lessens with increasing distance in the relationship between plants. Breeding would benefit from access to traits inherent in sexually noncompatible plants. Biotechnological techniques such as in vitro fertilization and embryo rescue (the excision and culture of embryos on nutrient media) have been employed to overcome incompatibility barriers, as have somatic hybridization and DNA technologies. Somatic hybridization involves enzymatic removal of walls from cells of leaves and seedlings to furnish individual naked cells, that is, protoplasts, which can then be fused to produce hybrids. Similarity of membrane structure throughout the plant kingdom permits the fusion of distantly related protoplasts. Cell fusion may lead to nuclear fusion, resulting in amphi-diploid somatic hybrid cells. Fusion products of closely related yet sexually incompatible plants have been grown to flowering plants; the most famous example is the potato + tomato hybrid = pomato (Solanum tuberosum + Lycopersicon esculentum). DNA technologies enable the isolation of desirable genes from bacteria, plants, and animals (genes that confer herbicide resistance or tolerance to environmental stress, or encode enzymes and proteins of value to the processing industry) and the insertion of such genes into cells and tissues of target plants by direct or indirect uptake has led to the genetic transformation of plant cells. The regeneration of transformed plant cells and tissues results in new and novel genotypes (transgenic plants). Contrary to hybrids obtained by cross-pollination, such plants are different from their parent by only one or two single, defined traits.