International Journal of Molecular Zoology, 2025, Vol.15, No.2, 90-100 http://animalscipublisher.com/index.php/ijmz 92 2.2 Genetic characteristics of egg production performance The egg-laying performance of laying hens includes annual egg production, egg weight, eggshell quality and egg production persistence, etc. Compared with growth traits, egg-laying traits generally have lower heritability and are more influenced by the environment. For example, the heritability of egg production is usually between 0.1 and 0.3, while that of egg weight is slightly higher, approximately 0.3 to 0.5 (Fu et al., 2023). This indicates that the genetic progress achieved solely through traditional phenotypic selection is limited. Egg production performance is controlled by the complex endocrine regulatory network of the hypothalamic-pituitary-ovarian axis (HPG axis), and its genetic basis involves numerous endocrine-related genes. Candidate gene research and association analysis have identified a batch of functional genes that affect egg production, including gonadotropin-releasing hormone (GnRH), neuropeptide Y (NPY), follicle-stimulating hormone receptor (FSHR), prolactin (PRL) and its receptor (PRLR), estrogen receptor (ESR), follicle-stimulating hormone receptor (GNRHR), etc. Among them, the polymorphism of the prolactin gene (PRL) and its promoter is closely related to the nesting behavior and egg-laying cycle of laying hens. Variations in the FSH receptor gene (FSHR) may affect follicular development and ovulation frequency (Kulikov et al., 2023). Among local chicken breeds in China, new genes related to egg production at 500 days of age, such as TFAP2D and THY1, have been identified through genome-wide association analysis, suggesting that these genes are involved in the regulation of ovarian function (Wang et al., 2024). In addition, there are a large number of QTL reports on egg-laying traits: According to the chicken QTL database, 540 QTLS are related to egg production, 56 QTLS are related to the age at first birth, and 393 QTLS are related to egg weight. However, these QTL intervals are often large, and the identification of candidate genes is not precise enough. 3 The Types and Principles of Hybrid Breeding 3.1 Principle of heterosis The phenomenon where the hybrid offspring perform better than the average of their parents is called heterosis or hybrid vigor. Among domestic chickens, the hybrid F₁ generation often outperforms purebred parents in terms of growth rate, egg production, disease resistance, etc. This is the fundamental reason why hybridization is widely adopted in the poultry industry (Ahmed et al., 2020; Gilyazova et al., 2025). The genetic mechanisms of heterosis mainly include the dominant effect theory, the superdominant theory and the superior theory, etc. Modern molecular research indicates that these three mechanisms may jointly act on different traits. For example, in terms of egg-laying traits, studies have found that the effect of positive superdominance is significant: the annual egg production of some laying hen strains with hybrid F1 is on average more than 5% higher than that of their parents. The significant positive hybridization effect indicates the presence of superdominant components (Wang et al., 2022). On the other hand, for growth and meat production traits, the cumulative effect of dominant alleles plays a major role, that is, through hybridization, the adverse effects of inferior alleles can be "masked", allowing the offspring to fully exert their growth potential. In practice, the magnitude of heterosis also depends on the genetic differences between the parents. Combinations of varieties with moderate genetic distance and high compatibility tend to produce the greatest heterosis (Mokoena et al., 2024). For instance, although there is a significant genetic difference when high-yielding laying hens are crossed with broilers, the hybridization in a single trait may not be as obvious as that between laying hen strains, because there is an antagonistic association between meat production and egg production. Based on this, breeders evaluated the hybridization effect of strains by measuring general combination ability (GCA) and specific combination ability (SCA) (Egahi, 2020). In the history of domestic chicken breeding, hybrid vigor has been "dissipated" in the separate selection and breeding of laying hens and broilers: broilers focus on growth and body size, while laying hens focus on reproduction and egg weight, each accumulating different dominant alleles. Through hybridization, these differentiated favorable genes can be expressed in the offspring, thereby demonstrating superior performance in comprehensive traits compared to the parents (Cendron et al., 2025).
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