Technical Abstract: The genome of the pig has been manipulated by selective forces ever since it became a species. Natural selection first modified reproductive and survival/fitness traits. Upon domestication of the pig, man also applied selective pressures concurrently with the ongoing forces of natural selection by breeding pigs with particular characteristics that were esthetically pleasing to the breeder. Some producers fancied various colored patterns while others placed greater emphasis on conformation or even behavioral disposition of the pig. This selective breeding process is partially responsible for the various breeds of pigs available today. After numerous generations of selection for the same physical attribute, some traits within each breed have become fixed. Current differences between breeds of pigs are accumulated responses of the genome to the unique selective pressures exerted on that population of swine. Many of these phenotypic differences have been discussed in Chapter 1. The number of unique genes which make up a mammalian genome has been estimated to be 50,000 to 100,000. Different forms of a particular gene can exist in a population and they are referred to as alleles. These phenotypic differences between breeds are due to differences in the frequency of specific alleles present in each breed.