Virulence of microbes or susceptibility of man and animals depends on the host parasite relationship. Different strains of the same species of animals, may show considerably different responses when they are exposed to a pathogenic organism. By crossbreeding of such strains, the natural resistance was first investigated under genetical control by Webster, on the relationship between the mouse and Salmonella enteritidis or B group of arboviruses, and the inheritance pattern of the resistance was shown to be explaind by the simple Mendelian law, being controlled independently by a main single dominant gene. The resistance controlled by dominant or recessive genes was also elucidated in infection with other viruses such as myxoviruses, mouse leukemia viruses, etc.
An inbred strain of DKI mouse which is highly and uniformly susceptible to the Salmonella enteritidis infection has been established in our laboratory. The development of a coisogenic strain having a resistant (R) gene of more resistant C3H/He strain on the DKI genetic background was made to evidence a single gene control for the resistance. The offsprings from mating of DKI × C3H/He hybrids to DKI were mated again to DKI strain, and the same procedures were made in subsequent generations. During the repeated backcrossing to DKI, R gene was selected by progeny test in each generation. After 10 generations of backcrossing, a coisogenic strain (DKI-R) which has R gene on genetic background of DKI was established by the incrossing of the BC (backcross) -10 mice.
The progeny test of BC-1 to BC-10 mice showed that about a half of the mice were heterozygous for R gene. And also, about a half of mice from mating the mice determined as heterozygous for R gene and DKI mice were resistant to the test challenge with Salmonella enteritidis in each generation. Thus, a single gene control of the resistance was demonstrated, though some accessory control by other genes is possible. To determine the phenotype of this gene, reticuloendothelial function and antibody production of DKI-R mice are being studied in comparison with the background DKI mice.

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