International Journal of Molecular Veterinary Research, 2025, Vol.15, No.1, 13-21 http://animalscipublisher.com/index.php/ijmvr 14 Genomic selection (GS) offers a revolutionary approach to breeding for complicated traits such as disease resistance. With the application of genome-wide molecular markers to predict breeding values, GS allows early and accurate selection of superior individuals with major and minor gene effects at the same time. GS has the capability to enhance genetic improvement significantly, decrease generations in poultry breeding, and facilitate multiple trait improvement at once, such as production traits and resistance to disease. Research on the use and optimization of GS is critical to comprehend its ability to enhance the effectiveness, precision, and sustainability of chicken breeding programs. 2 Genetic Basis of Disease Resistance in Chickens 2.1 Definition of disease resistance-related traits and methods for phenotypic measurement Disease resistance traits in chicken are the ability to prevent, suppress, or overcome infection by pathogens such as bacteria, viruses, and protozoa. Disease resistance traits are normally measured in terms of survival rates after infection, titers of pathogens in tissues, titers of antibodies, and clinical signs (e.g., tumour development, organ lesions) after challenge by experimental procedures. For example, resistance to Salmonella Pullorum is assessed by survival and bacterial carriage in the spleen after oral challenge, while resistance to Marek's disease is quantified by tumor incidence and survival upon infection (Li et al., 2018). Molecular diagnostics, serology, and direct challenge tests are typically used for phenotypic testing (Ding et al., 2025). 2.2 Heritability estimation and discovery of disease resistance-related genes Heritability of disease resistance traits in chickens is generally moderate, which provides encouragement for the potential for genetic improvement. For instance, heritability estimates for survival and carrier state after Salmonella Pullorum infection are 0.09 to 0.32, depending on line and trait measured (Li et al., 2018). Genome-wide association studies (GWAS) and SNP analyses identified significant resistance genes, such as MYH7, ATP2A3, CACNA1S for resistance to Salmonella, and PTPN13, TTF2, DLG2, CDH5 for resistance to avian leukosis (Ding et al., 2025; Li et al., 2025a). The major histocompatibility complex (MHC), especially the B locus, is repeatedly linked to resistance to multiple diseases, e.g., Marek's disease and infectious bronchitis (Miller and Taylor, 2016; Silva and Gallardo, 2020). A few of the other major genes include Nramp-1, IFN, Mx, and MyD88, which are involved in immune responses (Figure 1) (Dar et al., 2018; Gul et al., 2022). Figure 1 The host edited-genome and active immune responses of chicken during pathogen attack. Usually, chicken antigen presenting cells (dendritic cell, macrophage, B cell) engulf, digest, and present pathogen antigen on the cell surface in conjunction with an MHC molecule. The MHC/peptide complex stimulates the TCR and activates CD4+ and CD8+ cells. Accordingly, cytokines are produced, cell-signaling pathways such as JAK/STAT are activated, and a disease resistance state is developed inside the chicken that counters the pathogen virulence (Adopted from Gul et al., 2022)
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