The resolution of genes that determine resistance to disease is described using chicken lines maintained at the Avian Disease and Oncology Laboratory (ADOL). This description includes a summary 1) of existing selected and inbred lines differing for resistance to viral-induced tumors, i.e., Marek's disease (MD) and lymphoid leukosis (LL), and of the use of inbred and line crosses to define relevant disease-resistant genes, e.g., TV, ALVE, B, R, LY4, TH1, BU1, and IGG1; 2) of the development of TVB*/ALVE congenic lines to establish the affects of endogenous virus (EV) expression on resistance to avian leukosis virus (ALV), and methods to detect ALVE expression; 3) of the development of B congenic lines to define the influence of the MHC on MD resistance and vaccinal immunity, for producing B antisera, and for evaluating DNA sequences of Class I and II genes; and 4) of the current development of 6C.7 recombinant congenic strains (RCS) to define the role of non-MHC genes influencing susceptibility to MD and LL tumors, immune competence, and epistatic effects of genes. The procedures of pedigree mating, to avoid or maintain inbreeding, and of blood-typing, to ensure genetic purity of the lines, are also described.

The first standard nomenclature for the chicken (Gallus gallus) major histocompatibility (B) complex published in 1982 describing chicken major histocompatibility complex (MHC) variability is being revised to include subsequent findings. Considerable progress has been made in identifying the genes that define this polymorphic region. Allelic sequences for MHC genes are accumulating at an increasing rate without a standard system of nomenclature in place. The recommendations presented here were derived in workshops held during International Society of Animal Genetics and Avian Immunology Research Group meetings. A nomenclature for B and Y (Rfp-Y) loci and alleles has been developed that can be applied to existing and newly defined haplotypes including recombinants. A list of the current standard B haplotypes is provided with reference stock, allele designations, and GenBank numbers for corresponding MHC class I and class IIbeta sequences. An updated list of proposed names for B recombinant haplotypes is included, as well as a list of over 17 Y haplotypes designated to date.

Publications in which chickens of different B haplotypes were studied for differences in disease resistance or productivity traits are reviewed. The most prominent effects on diseases are those involving tumors, but other examples involving autoimmune disease and microbial infections not resulting in neoplasia or autoimmunity are also cited. Each referenced disease paper is briefly defined with regard to: population used, B alleles present, and the most resistant B types. Studies citing B haplotype influences on productivity and reproductive fitness traits are summarized and the most desirable B genes in each referenced population are given. Plausible mechanisms of the B haplotype's influence on the traits are briefly discussed. Based on the evidence reviewed for disease resistance and productivity traits and the central role of B-complex genes in immune function, it is concluded that poultry breeders should develop B-genotype information in their base breeding populations and use those types yielding optimal performance.