Testing mares and stallions can help ensure foals are born free or at low risk of some genetic diseases.

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Genetics refers to the study of genes and the way traits of conditions are passed down from one generation to another. Genomics, on the other hand, describes the study of all genes (the genome) including interactions of genes with each other and the environment. Although much of the genetic and genomic research done in Thoroughbreds is applied to racing performance, the full breadth of application of genetic and genomic research goes beyond that of faster horses.

Genetics and genomics allow for a more complete understanding of both simple and complex diseases. From a genetic perspective, simple is a term used to describe a disease that follows a single gene pattern of inheritance. These diseases are controlled by one gene, with other genes and outside factors having very little influence (i.e., the presence of the gene = disease). Diseases inherited this way are typically qualitative, where an animal either has the disease or it doesnt (i.e., lethal white foal syndrome).

Complex diseases, on the other hand, are usually controlled by not one, but many different genes and are often affected by environmental factors, such as nutrition and living conditions (e.g., cervical vertebral stenotic myelopathy, or wobbler syndrome). This combination of both genetic and environmental factors results in complex or multifactorial diseases. Basically, three different scenarios determine the manifestation of a complex disease:

As a result, complex genetic diseases can be extremely difficult to diagnose early and/or prevent using tradition methods such as pedigree analyses and veterinary evaluations. In some instances, a simple disease might even be classified as complex based on the inability of epidemiological studies and pedigree analyses to find common factors among cases.

Hydrocephalus, for example, is a developmental disorder that often results in stillbirth of foals and dystocia (difficult birth) in dams. Possible causes of the defect in horses could not be prove based on field data and pedigree analyses suggested the disorder to be complex. With this in mind, a genomic scan of 82 horses (13 cases and 69 controls) was performed and a small section of the genome was identified. Genomic sequencing was then performed on 10 horses (four cases and six controls) and the genetic cause of the disorder was pinpointed.

Figure 1

Two copies of a mutation that changes a "C" nucleotide to a "T" nucleotide results in hydrocephalus.

Photo: Equine Disease Quarterly

Ultimately, two copies of a mutation that changed a C nucleotide to a T nucleotide (Figure 1) resulted in the disorder. Although previously believed to be a complex disease, genetic and genomic methods were able to prove that the disorder was, in fact, simple, leading to the development of a genetic test that can help breeders avoid the disorder.

It is important to note the difference between a genetic test of a simple disease, such as hydrocephalus, and a genetic test for a complex disease (e.g., osteochondrosis). Genetic tests for simple diseases can confirm or rule out a genetic condition; however, genetic tests for complex diseases only help to determine an individuals chance of developing a genetic disorderan important distinction when genetic tests are used to help make breeding decisions. In either scenario, genetics and genomics in Thoroughbreds have far-reaching potential beyond that of breeding and selecting faster horses.

Understanding diseases caused by a single gene as well as complex diseases caused by multiple genes and the environment can lead to early diagnosis and targeted treatments. While the list of reasons a racehorse never reaches its potential might seem endless, genetics and genomics provide an opportunity to cross certain disorders off that list, thereby helping to eliminate or reduce the occurrence of those diseases.

CONTACTBrandon D. Velie, MS, PhDbrandon.velie@slu.seSwedish University of Agricultural Sciences Department of Animal Breeding and Genetics, Uppsala, Sweden

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyds, London.

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Genetics and Genomics in Racing: Speed Isn't Everything - TheHorse.com