Experiments were planned to study the influence of selection upon natural resistance to fowl typhoid in the chicken, and to determine something about the genetic nature of differences in resistance.

Using a pathogen of constant virulence and a uniform method of infection, it was determined that degree of infection influenced the mortality in unselected chicks. Both total mortality and rate of mortality were increased by the heavier doses.

Four generations of selection for resistance to a standard dose of fowl typhoid bacteria resulted in a decided decrease in mortality in the selected population. The mortalities observed in the chicks of the selected generations, S1 to S4, were respectively 40, 29, 15 and 23 percent. In the unselected (control) population the respective mortalities were 90, 93, 86 and 86 percent. The increased mortality in the selected S4 population was due to unavoidable chilling of the last two groups of chicks tested in that generation.

An analysis of the breeding records of the selected population shows degrees of inbreeding over 30 percent for some of the birds of this stock, but so far it is difficult to evaluate the part that the inbreeding has played in bringing about the increased resistance of the selected stock.

Reciprocal crosses of selected by unselected birds demonstrated that the male as well as the female transmits resistance to the offspring. These results show that a passive transfer of immunity was not a great, if existent, factor in the enhanced resistance of the selected progeny.

F₁, F₂ and backcross progenies from a parental cross of resistant with highly susceptible unselected birds indicated the presence of genetic factors for resistance.

No evidence was found of a linkage between major factors for resistance or susceptibility and the gene for inhibition of plumage color (I), or its allelomorph (i). Nor was there evidence of major sex-linked factors for resistance.

With the exception of one cross (selected males with unselected females) significant differences in mortality were not observed between males and females.

Inherent resistance to fowl typhoid is undoubtedly controlled by multiple factors some of which show dominance or partial dominance. Because of the complexities introduced by environmental variations and the interrelations of host and pathogen, the genetic analysis of resistance to fowl typhoid must await the development of inbred lines which react in a uniform and reasonably constant manner to infection with this bacterium.