Horses don't come with barcodes. There are few obvious outward signs of true genetic merit, even to the experienced horseman. The phenotype that can be measured is a function of genetic and environmental effects. A mediocre horse with good training, or one that has been campaigned strategically, can appear better than he actually is, while an incredibly talented mount may find himself in an environment in which he will never have the opportunity to fulfill his potential. However, selection for physical attributes such as beauty of the head, height at the withers, or many conformational traits may be easier than for performance ability, as these traits generally have a lower environmental component.
The term heritability describes how much of a population's variation for a trait is due to environmental factors, and how much is due to the summed effects of genes responsible for the trait in question. The fraction of total variation due to genetic effects is defined as heritability, and is expressed as a number between 0 and 1. It is important for breeders to remember that heritability is a population parameter, and is not concerned with individual horses or individual genes. Numerous genes influence most performance traits. Money earned by a racehorse, for example, is not due to the effects of a single gene. More than likely, superior racing performance is due to many genes working together to enhance aspects of racing ability such as lung capacity, desire to win, bone strength, metabolic efficiency, and more.
Heritabilites for conformation traits are generally moderate to high, as are those for gait characteristics.1-4-1 Performance traits, such as racing, dressage, or jumping ability, generally have low heritabilities. For the breeder, a trait with high heritability means that by using animals who excel for that trait, progress will be made rapidly. If heritability is low, however, progress may be better made by enhancing management, because much of the total variation is due to environmental effects rather than to genetic effects.
Also of importance when breeding horses are genetic correlations between and among traits. When breeders select for one trait, other traits may be desirably or undesirably associated. If breeders attempt to select for two negatively correlated traits, genetic progress may be hindered, depending on the strength of the association.
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