نوع مقاله : مقاله پژوهشی
عنوان مقاله English
نویسنده English
Introduction: Precise genetic parameters form the scientific cornerstone of effective breeding programs. In horses, morphological (conformation) traits directly impact performance, health, breed classification, and economic value. However, considerable inconsistency and heterogeneity exist in reported estimates of heritability (h²) and genetic correlations (rg) for these traits across individual studies. These discrepancies hinder the development of reliable and generalizable breeding strategies. Meta-analysis offers a robust statistical framework to integrate findings from multiple studies, providing more precise and generalizable pooled estimates while quantifying heterogeneity. Therefore, this study aimed to conduct a comprehensive meta-analysis to synthesize existing research and deliver consolidated, reliable estimates of genetic parameters for key morphological traits in horses.
Materials and Methods: A systematic literature search was conducted in the PubMed database using combined keywords related to horses (horse, equine), morphological traits (conformation, morphology, withers height, body length, etc.), and genetic parameters (heritability, genetic correlation). After initial screening, stringent inclusion criteria were applied: 1) the study had to report numerical estimates of h² or rg; and 2) each trait had to be investigated in at least three independent studies to allow for meaningful meta-analysis. From an initial pool of 34 relevant studies, data were ultimately extracted from 25 eligible publications. This included estimates for 14 morphological traits and 3 key pairs of genetic correlations. The meta-analysis was performed using the Random-Effects Model (REM) in R software (version 4.4.3) with the metafor package. The REM was chosen over the fixed-effects model as it explicitly accounts for between-study variance (τ²), making it more suitable for the anticipated methodological and population heterogeneity across studies (e.g., different breeds, measurement protocols, and genetic models). Studies were weighted using the inverse-variance method. Heterogeneity among study estimates was statistically assessed using Cochran’s Q test and quantified with the I² statistic. Publication bias and funnel plot asymmetry were examined using Egger's linear regression test and the non-parametric Trim-and-Fill method. Where significant heterogeneity was detected (p<0.05 for Q), moderator (meta-regression) analysis was employed to identify potential sources, such as the time period of the study.
Results and Discussion: The meta-analysis yielded robust pooled estimates. Heritability for the 14 morphological traits ranged from low to high (0.08 to 0.75). Key structural traits exhibited moderate to high heritability, such as withers height (h² = 0.55, 95% CI: 0.39–0.72) and croup height (h² = 0.75, 95% CI: 0.61–0.89). In contrast, subjectively scored gait traits like walk showed low heritability (h² = 0.08). Genetic correlations among major biometric traits were consistently positive and strong. For instance, the correlation between withers height and chest circumference was rg = 0.67 (95% CI: 0.53–0.84), and between cannon bone circumference and chest circumference was rg = 0.72 (95% CI: 0.50–0.93). Statistically significant heterogeneity among studies was detected only for the trait withers height (Q-test p<0.05, I² = 48.5%). This heterogeneity was successfully resolved through moderator analysis, identifying the study period as a significant source. After adjusting for this temporal effect, the heterogeneity became non-significant (I² = 18.8%). For all other traits and for the genetic correlations, heterogeneity was negligible (I² ≈ 0%), indicating high consistency among the included studies. No significant publication bias was confirmed for the majority of traits based on Egger’s test and Trim-and-Fill analysis. The moderate-to-high heritability estimates confirm a substantial additive genetic component for most conformational traits, indicating significant potential for genetic improvement through selection. This aligns with breed-specific studies showing tangible morphological change over decades of selective breeding (e.g., in the Murgese breed). The strong positive genetic correlations suggest shared genetic architectures and coordinated development, enabling efficient multi-trait selection. These correlations justify the use of advanced genetic evaluation models, such as multi-trait models or reduced-rank factor analytic models, which can enhance accuracy and computational efficiency. However, the findings necessitate a nuanced and balanced approach to breeding. The low heritability of some subjectively scored traits underscores the critical need for standardized, objective phenotyping protocols to minimize environmental noise and evaluator bias. Furthermore, the significant heterogeneity observed for withers height—explained by the study period—highlights the potential influence of evolving management practices, genetic trends, or measurement techniques over time, reinforcing the value of meta-analysis in contextualizing historical data. Most importantly, while the genetic parameters indicate potential for progress, they must be applied with caution. Evidence from studies on certain breeds (e.g., Rhenish German Coldbloods) reveals that strong positive genetic correlations can exist between desirable morphological traits (e.g., large bone circumference) and debilitating diseases like Chronic Progressive Lymphedema (CPL). This serves as a critical warning that intense single-trait selection for morphology, without considering correlated health responses, can have severe unintended consequences. Therefore, the successful exploitation of the genetic potential identified in this meta-analysis mandates the definition of balanced, multi-dimensional breeding objectives. These objectives must explicitly integrate traits for performance, durability, health, and welfare alongside conformation, and must be supported by rigorous management of inbreeding to preserve genetic diversity.
Conclusion: This meta-analysis provides consolidated, reliable estimates of genetic parameters for equine morphological traits, confirming their moderate-to-high heritability and strong positive inter-correlations. These results offer a robust scientific reference for designing and optimizing breeding programs across diverse horse breeds. To translate this genetic potential into sustainable improvement, breeding strategies must employ advanced genetic evaluations, precise phenotyping, and—above all—holistic breeding goals that ensure the long-term health, functionality, and genetic vitality of equine populations.
کلیدواژهها English
