The Genetic Relationship between Mortality from Induced and Spontaneous Lymphomatosis

POULTRY breeders have endeavored to increase resistance to lymphomatosis by hatching chicks from families which seem to be relatively resistant to this disease. They sometimes are handicapped in their efforts, particularly when cockerels or pullets are used for breeding, because a low incidence of the disease prior to the time matings are put together makes it difficult or impossible to accurately identify families having the greatest resistance. We may assume that most efficient selection is possible when the average mortality is higher than 20 to 30 percent (Fisher, 1937, pp. 235–238). In the absence of spontaneous mortality of this magnitude, the knowledge that such high levels of mortality may be desirable is quite useless unless the breeder is able to deliberately increase the exposure so that the incidence of the disease reaches an optimum level. That such an increase in exposure to the leukosis complex of diseases may be . . .