The aim of selection is to increase the frequency of desired alleles and decrease the frequency of undesired genes in a population, ideally producing animals that breed true for the genotypes and phenotypes selected for.
One influence on the effectiveness of selection on gene frequency changes is the initial gene frequency in a population. Consider two alleles at locus A: A1 is wanted and A2 is not. This graph plots the frequency of A2 in each successive generation, showing the effect of selection against that unwanted allele A2 over many generations:
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