International Journal of Molecular Zoology, 2025, Vol.15, No.2, 90-100 http://animalscipublisher.com/index.php/ijmz 96 systems has led to a continuous increase in laying hen production over the past few decades. In the 1950s, the annual output of laying hens was less than 200. Nowadays, the output of excellent hybrid laying hens can reach more than 320 (Fu et al., 2023). 5.2 Development of dual-purpose chicken breeds and balanced trait breeding With the emphasis on animal welfare and sustainable development concepts, breeders have begun to attempt to breed dual-purpose chickens, that is, chicken breeds that can produce both meat and eggs, in order to reduce reliance on issues such as culling of male chicks (de Haas et al., 2021). The breeding objective of dual-purpose chickens is to have both medium to high egg-laying capacity and meat value within the same strain. The difficulty lies in the fact that there is usually a negative correlation between meat production traits and egg-laying traits. However, hybrid breeding offers a possible path to achieving this balance. One approach is to conduct cross-group hybridization between broiler chicken strains and laying chicken strains to cultivate new strains that are versatile. Another direction in the breeding of dual-purpose chickens is to select and breed on the basis of existing local breeds to achieve a performance level that can be used for both purposes. In this regard, some countries in Africa and Asia are attempting: through crossbreeding and selection of local chickens with high-yielding laying hens and broilers, hybrid populations with better meat and egg production under local conditions have been cultivated (Mokoena et al., 2024). It should be emphasized that the promotion of dual-purpose chickens also depends on market acceptance and economic benefits. At present, the production efficiency of professional broilers and laying hens is extremely high, while dual-purpose breeds are slightly inferior in both meat and egg production. 5.3 The impact of hybridization on egg quality Hybrid breeding not only increases yield but also has a certain impact on egg quality, including eggshell strength, egg white consistency, egg yolk color, etc. In hybrid combinations, it is often possible to take advantage of the different egg quality strengths of the parents to improve or at least not reduce the egg quality of the offspring. As long as the parents are carefully selected, hybridization will not damage the quality of the eggs; instead, it can combine the advantages of all parties. It is particularly effective in improving the quality of eggshells and can reduce the rate of broken eggs. In addition, hybridization can also be used to change the color and special components of eggs. If the genes of blue-shelled eggs are introduced into laying hen strains, high-yield blue-shelled laying hens can be bred, which has been commercialized both at home and abroad. For instance, by introducing high lutein genes through hybridization, the color index of egg yolks can be increased, and deep yellow egg yolks that consumers love can be produced. It should be noted that the quality of eggs is also greatly affected by feed and management. In the breeding of hybrid varieties, reasonable nutrition and environment should be provided to fully exert their genetic potential (Mokoena et al., 2024). 6 Challenges and Improvement Directions in Hybrid Breeding 6.1 The difficulty of constructing hybrid systems and genetic complexity Although hybrid breeding has achieved remarkable results in the improvement of domestic chickens, its implementation and optimization are not without challenges. Building a stable and efficient hybrid system requires long-term investment and technological accumulation. A matching system is generally composed of multiple pure lines, and the cultivation and maintenance of these pure lines themselves are difficult and costly. From a genetic perspective, the mechanism of heterosis is complex, and the degree of dominance of different traits varies greatly. Not all important traits can be optimized through a single hybridization (Ahmed Soliman et al., 2020). Furthermore, there are extensive gene-gene interactions (superior effects) in the genomes of hybrid offspring, making it difficult to fully predict the performance of the offspring. The genetic association between purebred and hybrid breeds also poses challenges to breeding, and the genetic trade-offs among different traits are also thorny issues (Miyumo et al., 2023). 6.2 Coordinated development of conservation and breeding The extensive use of high-yield commercial strains in hybrid breeding may lead to the loss of local varieties and genetic diversity. This is one of the challenges that the poultry industry must confront. When promoting hybrid
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