EVALUATION OF MAJOR GENES AFFECTING RESISTANCE TO DISEASE IN POULTRY

The results from experimental studies about genetic factors ('major ge nes') involved in disease resistance and their effects in different ge netic backgrounds are reviewed and discussed. The major genes consider ed are (1) Tv-A and Tv-B, gene loci controlling resistance to infectio n from avian leukosis virus (ALV), (2) B blood groups which represent the major histocompatibility complex (MHC) of the domestic fowl and ar e indicators of genetic differences in resistance to Marek's disease ( MD) and, only tentatively, (3) endogenous viral (ev) genes which can i nfluence reactions to leukosis infection. For these experiments purebr ed and crossbred progeny from three genetically different experimental White Leghorn lines have been used extensively The results concerned with leukosis are based on a comparison between groups of chickens whi ch were homozygous resistant or susceptible to ALV infection. The vari ability in the amount of damage from leukosis points to interactions b etween the genotype of resistance to ALV infection and genetic charact eristics of the line or lines involved. In this respect the negative i nfluence of ev 6 on the immune response to ALV infection could play a role. To study the association of B blood groups and resistance to MD, an experimental line which still segregated at the B locus (BS B13, B 14 and B21) was crossed with two other lines which were homozygous, re spectively, for B2 and B15. Mortality caused by MD in birds carrying t he same segregating B blood groups showed a completely different patte rn in the two crosses. Rather unexpectedly, the difference in MD morta lity was largest between the two types of crossbred layers carrying B2 1, which points to an interaction between line and B blood group type. Further studies suggested that this interaction depends on genes with in the MHC for which the lines crossed might differ. The availability of information from different genetic groups allowed an evaluation of the importance of each 'major gene' in relation to the comparative eff ects of other factors influencing the trait under consideration. Apart from becoming aware of possible interactions resulting from genetic d ifferences between lines, simultaneous results from genetically differ ent experimental lines also provide a safeguard against overestimating or underestimating the potential of such 'major genes' for practical utilization in poultry breeding.