Livestock (1993) / Chapter Skim
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3 Measurement and Use of Genetic Variation
3 Measurement and Use of Genetic Variation
Pages 63-76
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From page 63... ...
In animal agriculture, manipulation of genetic variation by controlling reproduction (through selection and crossbreeding) has been the foundation for improving livestock populations.
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From page 64... ...
Subdivision into Breeds The subdivision of domestic species in response to different human preferences reached its zenith in the 1800s with the formation of breed societies in western Europe and in the regions colonized by western Europeans. The concept of breed identity rested on the idea that uniformity of type was an important goal in livestock production and could only be achieved by controlled matings involving animals of known parentage.
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From page 65... ...
In the U.S. beef cattle herd, for example, many animals express apparent phenotypic breed identity (usually as a result of color)
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From page 66... ...
Members of a breed share a common ancestry and selection history. For example, the Holstein breed of dairy cattle, developed from the Dutch Black Pied breed, have been selected to produce large quantities of relatively low-fat milk, while the jersey breed of dairy cattle, from the Channel Islands, has been selected to produce smaller quantities of richer, high-fat milk.
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From page 67... ...
A strong argument for breed preservation, then, is the need not just to maintain genetic diversity but also to maintain its accessibility in predictable source populations. FACTORS AFFECTING GENETIC VARIATION Allelic frequencies are altered by four forces: selection, migration or gene flow, mutation, and random genetic drift.
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From page 68... ...
Breeding goals have changed over the years, and artificial selection practiced by animal breeders has varied accordingly. Currently, a range of selection goals can be identified for almost all livestock species; the most pronounced differences are found for ruminants raised for their meat.
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From page 69... ...
When population sizes are small, random genetic drift can lead to loss of alleles owing simply to chance, particularly if they are present at low frequencies. A practical consequence is that the number of breeding individuals maintained in a preserved population must be sufficiently large to avoid the potential for decreased genetic variation resulting from genetic drift.
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From page 70... ...
Selection has been effective, and the industrialized stocks are now clearly differentiated from the original stocks. The possibility of drastic changes in production and marketing conditions, and the need to return to, or sample genes from, Reindustrialized stocks, cannot be totally discounted.
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From page 71... ...
, and they provide strong justification for the preservation and characterization of the genetic resources of poultry, pigs, and cattle. Nonindustrial Populations For extensively managed ruminant species raised for meat and fiber, and for nonindustrialized populations in general, germplasm .~ .
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From page 72... ...
Generally, the efficiency of grazing ruminants can be increased by improving their ability to adapt to their environment. The existence of many distinct populations, each adapted to a particular environmental niche, can best ensure the capability to improve meat and fiber ruminant species.
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From page 73... ...
The continued adaptation of grazing ruminants to these changing environments can be achieved by selection, but it is often accomplished more efficiently by recombining several existing populations to form a new germplasm base consistent with the new production conditions. Thus, Sahiwal cattle from the Indian subcontinent have contributed to milk-producing stocks in Africa and Australasia; prolific Finnish Landrace sheep have contributed to the development of several new breeds worldwide for example, the Polypay sheep for use in improved grazing areas of North America; and Africander pairs, the number of different alleles that could theoretically be formed from even a short piece of DNA is extremely large.
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From page 74... ...
The genetic characterization of large numbers of indigenous breeds is costly and time consuming, but the evaluation of promising breeds is a critical part of livestock improvement efforts. The general use of unimproved or undeveloped germplasm as a source of genes for improvement is less likely for modern industrialized and intensively managed livestock than it is for improvement of nonindustrialized, extensively managed livestock, such as the grazing ruminants.
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From page 75... ...
The evaluation of these breeds, then, becomes a priority. The potential of preserved breeds to contribute to future production systems is also greater for the grazing ruminants because the global range in production environments encompasses such a wide range of conditions and because highly specialized, industrialized stocks do not yet exist for these species.
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