Thoroughbred Winners Through Quantitative Genetics

For more than 300 years, thoroughbred horses have been raised for a single purpose—to win at the racetrack. The origin of these horses can be traced to a small group that was imported to England from North Africa and the Middle East in the 1600s. The population of racing horses remained small until the 1800s, when horse racing became increasing popular; today there are approximately half a million thoroughbred horses worldwide.

Breeding and racing thoroughbred horses is a multibillion-dollar industry that relies on the premise that a horse's speed is inherited. Speed is not, however, a simple genetic characteristic such as seed shape in peas. Numerous genes and nongenetic factors such as diet, training, and the jockey who rides the horse all contribute to a horse's success or failure in a race. The inheritance of racing speed in thoroughbreds is more complex than that of any of the characteristics that we have studied up to this point. Can the inheritance of a complex characteristic such as racing speed be studied? Is it possible to predict the speed of a horse on the basis of its pedigree? The answers are yes—at least in part—but these questions cannot be addressed with the methods that we used for simple genetic characteristics. Instead, we must use statistical procedures that have been developed for analyzing complex characteristics. The genetic analysis of complex characteristics such as racing speed of thoroughbreds is known as quantitative genetics.

Although the mathematical methods for analyzing complex characteristics may seem to be imposing at first, most people can intuitively grasp the underlying logic of quantitative genetics. We all recognize family resemblance: we talk about inheriting our father's height or our mother's intelligence. Family resemblance lies at the heart of the statistical methods used in quantitative genetics. When genes influence a characteristic, related individuals resemble one another more than unrelated individuals. Closely related individuals (such as siblings) should resemble one another more than distantly related individuals (such as cousins). Comparing individuals with different degrees of relatedness, then, provides information about the extent to which genes influence a characteristic.

This type of analysis has been applied to the inheritance of racing speed in thoroughbreds. In 1988, Patrick Cunningham and his colleagues examined records of more than 30,000 3-year-old horses that raced between 1961 and 1985. They reasoned that, if genes influence racing success, a horse's racing success should be more similar to that of its parents than to that of unrelated horses. Similarly, the racing speeds of half-brothers and half-sisters should be more similar than the speeds of unrelated horses are. When Cunningham and his colleagues statistically analyzed the racing records for thoroughbreds, they found that a considerable amount of variation in track performance was due to genetic differences—racing speed is heritable. With the use of statistics, it is possible to estimate, with some degree of accuracy, the track performance of a horse from the performance of its relatives.

This chapter is about the genetic analysis of complex characteristics such as racing speed. We begin by considering the differences between quantitative and qualitative characteristics and why the expression of some characteristics varies continuously. We'll see how quantitative characteristics are often influenced by many genes, each of which has a small effect on the phenotype. Next, we will examine statistical procedures for describing and analyzing quantitative characteristics. We will consider the question of how much of phenotypic variation can be attributed to genetic and environmental influences and will conclude looking at the effects of selection on quantitative characteristics. It's important to recognize that the methods of quantitative genetics are not designed to identify individual genes and genotypes. Rather, the focus is on statistical predictions based on groups of individuals. More information on horse genetics, including the genetic basis of coat color, gene mapping, chromosomes, and genetic disorders

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