Genetic and permanent environmental variability of twinning in Thoroughbred horses estimated via three threshold models.

The twinning of foals, which typically results in miscarriage and poor performance in thoroughbred horses, is attributed to genetic and environmental factors according to a study analyzing 12,648 pregnancies from 2033 mares. This variability was assessed using three statistical models, which pointed to the possibility of a major gene contributing to twinning effects.

Objective and Methodology

• The goal of this study was to determine whether genetic factors and permanent environmental variables could affect the rate of twinning in Thoroughbred horses. This is of interest as twinning in horses often leads to miscarriage and low performance.
• To explore this, the research team analyzed 12,648 pregnancies from 2033 Thoroughbred mares born between 1929 and 1994 in 11 studs.
• Data processing was done using AI-REML (Average Information Restricted Maximum Likelihood) algorithm under three threshold animal models. Depending on the model used, variables such as the horse’s stud, the mare’s birth period, and their interaction were treated alternatively as fixed or random.

Comparative Analysis of Models and Results

• Model comparison was based on the deviances — a statistical measure used to compare the fit of different models to the observed data.
• Model 1 fixed the factors like stud, mare’s birth period, and the interaction of these two variables. Model 2 and Model 3 treated these variables as random factors, with the latter also treating the stud effects as random.
• The rate of twinning in the studied population was found to be 3.5%, including 443 twins and two triplets.
• The heritability of twinning was estimated by each model, varying from 0.24 (model III) to 0.29 (model I). This signifies the extent to which genetic factors can influence twinning rates. The repeatability of twinning ranged from 0.33 (model I) to 0.34 (model III), indicating the consistency of the phenomenon.
• A yearly evaluation was also done for phenotypic and genetic trends.

Conclusions

• Fluctuations observed in the average annual genetic effects could be due to the major gene’s segregation in the horse population, suggesting that a significant gene could influence the twinning rate.
• Although the models vary, they all indicated a fairly good level of heritability and repeatability for twinning, indicating a certain genetic predisposition to this problem in Thoroughbred horses.