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United States Department of Agriculture
Abstract
The animal characteristics of greatest importance to the breeder of live stock are those concerned with yields of the various animal products which find a ready sale in commerce. Meat, milk, eggs, wool and even speed are some of the more important things which breeders hope to improve both quantitatively and qualitatively. Unfortunately, production of these commodities depends upon a highly intercorrelated set of genetic, developmental and environmental factors which the breeder cannot separate and deal with individually as is customary in the usual genetical analysis. In fact the great majority of individual genes must be considered at the same time and in relation to the whole organism. To further complicate matters we lack a sufficiently accurate description of many animal characters to allow a satisfactory genetical analysis to be made even if no other impeding factors were present.
The work with small laboratory animals has pointed out the way by which heredity affecting such complex characters as fecundity, growth and conformation may be controlled. Yet a study of the present breeding methods in this country shows that there are few cases indeed where such methods are being employed. Almost every known system of breeding from the most intense to one in which random mating is limited only by natural selection can be found in some sections of this country. The majority of breeders use one of the many different line breeding systems accompanied by selection. Such a policy is sound in principle but it must be admitted that few breeders realize the difficulty of making consistent progress when phenotype of the individual is used as the measure of genotype. Only a few breeders inbreed closely enough to fix heredity for the various characters. On the other hand many breeders resort to crossbreeding. When this is systematically conducted with purebred parents and all hybrids are slaughtered the results may be very favorable. The production of hothouse lambs for eastern markets by mating Merino ewes to Southdown rams is a good illustration of this. The change from fine to coarse wool and vice versa in the northwestern intermountain region is another example of crossbreeding but in this case exactness of procedure and clarity of purpose are somewhat lacking. Many swine and cattle producers also frequently resort to crossbreeding as a temporary expedient in market live stock production. There is little tendency, however, to continue beyond the first cross. Experimental breeding of this nature with slow maturing animals is far too expensive to be taken lightly and the case is of course embarrassed by the well established pedigree system and the existence of powerful breed organizations. It is cheaper and easier to produce creditable specimens of established breeds than it is to create anything new.
Perhaps the greatest service the science of genetics has renedered to the breeder is the introduction of a new point of view. It has shown that many animal characters are not units of inheritance but are combinations of a large number of independent units. It has been demonstrated how such units can be transferred from one animal to another. That there is so little relation between the science and the craft of breeding is not entirely the fault of the science but is partly due to the complex economic situation in which we live. Breeders must realize profits from their business and the genetic methods involved are of relatively little consequence unless they are the ones which yield the greatest profit.
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