SummaryIncorporation of disease resistance into breeding programmes would be enhanced by adequate methods. General disease resistance instead of resistance to specific diseases might be a useful concept.Selection for specific mortality caused by Marek's disease, lymphoid leukosis and Newcastle disease is possible, but not desirable in practical breeding programmes. Viraemia post Marek's disease vaccination, tumour formation after Rous sarcoma virus injections and haemagglutination-inhibition titres post vaccination with inactivated Newcastle disease virus may be better indicators of resistance. However, knowledge of genetic resistance to these specific diseases has not yet produced much evidence for general mechanisms except the involvement of the B major histocompatibility complex in viral oncogenesis.Since immune responsiveness is controlled by genes within and outside the B complex, the search for genetically determined immune criteria is needed to define general resistance to disease. Selection for immunoglobulin levels or the antibody response to sheep erythrocytes are the first investigations in this area. Genetic variation in the cell-mediated response and in the humoral response to Salmonella pullorum, Escherichia coli and Newcastle disease virus has also been shown. Ultimately an index, based on humoral and cell-mediated immune responses, phagocytosis and markers of the B complex, should be composed indicating the general capacity to resist infectious diseases.Studies concerning the genetic relation between disease resistance and production traits are scarce. The negative genetic correlations between body weight, and antibody production and resistance to MD, may cause reconsideration of selection goals in broiler breeder programmes. Evaluation of selection methods for production traits can be expected awaiting knowledge of relationships with disease resistance, either specific or general.
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A.J. van der Zijpp (1983) studied this question.