Selection by the breeder of livestocks is aimed, as a rule, at improving the efficiency of converting external resources, particularly feed, into marketable products (meat, milk, eggs, wool, etc.), produced by crowded animals that compete for limited resources. These conditions of crowding and competition may be defined as the commercial environment. The genotype (gc) to be improved, namely, efficiency of utilization of external resources under commercial environment, cannot be selected directly by mass selection. The only way for its direct selection is to stock whole families, each in a separate pen, and to use the family mean as the unit of selection. Under conditions of family selection, competition may considerably increase the intra-pen variability, while having only a small effect on the pen’s mean (Jonsson, 1959). The relationship of the genotype (gj) selected by mass selection under competition situation (environment) to the genotype to be improved (gc) can be expressed as

R. Moav

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 3, Madrid, Spain, 415–419, 1974
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