International Journal of Molecular Zoology, 2025, Vol.15, No.2, 90-100 http://animalscipublisher.com/index.php/ijmz 93 3.2 Classification of hybridization methods According to the differences in the hybridization strains and generations involved, the hybridization methods of domestic chickens can be classified into several types. Among them, the hybridization of two varieties (strains) is the most basic form, that is, directly crossing two parent varieties for use in production. For instance, many commercial broiler chicken lines are formed by crossing two pure lines to create the final commercial generation. In the production of broilers and laying hens, three-line or four-line crossbreeding is often adopted. Take four-line hybridization as an example. First, four pure lines A, B, C and D are selected and bred: A×B hybridization produced paternal sire line, C×D hybridization produced maternal dam line, and then the parent hybridization lines were mated to obtain the final commercial generation (Jha et al., 2015). In the field of laying hens, three-line hybridization is also often adopted: two purebred lines are crossed as the paternal line, and then a third purebred maternal line is crossed. The commercial generation has the characteristics of high egg production and strong health. Backcrossing and reincarnation hybridization are also methods used in poultry improvement. Reincarnation hybridization involves alternating hybridization among multiple varieties to maintain heterozygous dominance within the population. The choice of hybridization methods depends on the breeding goals and the breeding system: if commercial generations are not used for seed breeding, it is advisable to boldly adopt multi-strain combinations to maximize hybrid superiority. For conservation or regional variety improvement, hybridization of two varieties combined with backcrossing is often used to balance the improvement of performance and the preservation of the gene pool (Jha et al., 2015). In recent years, the idea of molecular design hybridization has emerged, which is to predict the heterosis of different strain combinations based on genomic information and then arrange the hybridization plan accordingly. This is achieved by screening for polymorphisms in the whole genome of candidate parents and optimizing the parent combination using indicators such as heterozygosity and complementarity of key genotypes. It is reported that there are already hybrid vigor prediction models based on molecular markers for crops such as corn, and research in this field for poultry is also gradually unfolding (Egahi, 2020). 3.3 Parent-based choice and matching system construction strategy The selection of the parent strain is the key to the success or failure of hybrid breeding. The ideal parental combination should be complementary in genotype: that is, one makes up for the shortcomings of the other, so that the offspring achieve the best balance in the target traits. Taking broiler chicken breeding as an example, the selection of the paternal strain focuses on weight gain and feed conversion ratio, while the maternal strain balances egg production and early growth, so that the maternal generation can produce sufficient breeding eggs without growing too slowly (Jha et al., 2015). First, establish several genetically diverse basic strains, and conduct pure strain selection within each strain to achieve a high level of genetic stability in certain traits (for example, one strain focuses on growth rate and selects large individuals in each generation; another strain selects reproductive capacity). Then, hybridization tests were conducted according to the predetermined combinations. The compatibility of each strain was evaluated by measuring the performance of the first generation of hybridization, and the best parent combination was screened out (Egahi, 2020). When making parent-based choices, attention should also be paid to the accumulation of close friends and the size of the effective group. Long-term closed breeding of purebred parent lines is prone to inbreeding decline, thereby affecting the hybridization effect. Therefore, new bloodlines need to be introduced regularly or family rotation should be adopted to maintain pure-line healthy genetic diversity (Cendron et al., 2025). Meanwhile, in order to improve the overall efficiency of the matching system, the reproductive performance of the parent parents, especially the egg-laying and hatching performance of the mother parents, must be good to produce sufficient and high-quality hybrid breeding eggs (Jha et al., 2015). This requires that when selecting parents for breeding, in addition to the target traits, attention should also be paid to their reproductive-related traits.
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