It occassionally occurs in breeding practice that optimum phenotype of the parental population differs from optimum phenotype of the offspring population. The outstanding example of this is found with respect to the traits growth-rate and mature body size in meat producing livestocks (e.g. broiler poultry, beef cattle). In this case, rapid growth-rate, is a major avenue for increased efficiency in the offspring animals raised for meat. The strong genetic correlation between growth-rate and mature body size, however, means that selection for growth-rate will increase mature body size as well (Kinney, 1969; Cartwright, 1979). This reduces efficiency of the overall enterprise by increasing growth and maintenance costs of the reproductive population (Cartwright, 1979). The use of specialized sire lines, selected intensely for rapid growth rate and used for crossing with appropriate dam lines provides a partial differentiation of dam-line and offspring phenotypes (Moav, 1966). An elegant advance on this approach, presently being implemented in poultry, is the incorporation of a recessive dwarf gene in the dam-line (Guillaume, 1976). The effect of the gene in the offspring is eliminated by the use of an appropriate sire line, homozygous for the alternative normal allele. Major genes for dwarfism, however, are not found in other meat producing livestocks, and in general major genes effecting economic traits are rare. Thus, this solution is not widely applicable. Growth rate, however, is a polygenic character, and it is reasonable that segregation at some of the relevant loci involves recessive alleles for reduced growth rate (mature body weight). These alleles are the anlaog of the recessive dwarf gene in poultry. Accumulation of such alleles in a dam line would reduce female mature body weight while crossing the dam line with a sire line selected intensively for growth-rate and hence close to fixation for the alternative dominant alleles would eliminate the effects of the recessive alleles in the offspring. Development of such a dam line requires breeding methods that increase the frequency of recessive alleles for slow growth-rate in a population, while simultaneously increasing or at least maintaining the frequency of codominant and recessive alleles for rapid growth rate. Breeding procedures that can achieve these goals are presented
in this paper.
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 7. Symposia (1), , 424–428, 1982
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