Inbreeding, also known as line breeding, is the practice of breeding animals that are closely related. It is a time-honored technique used in the equine industry to ensure desirable traits are passed on from generation to generation.
However, when it comes to thoroughbreds, inbreeding has put the breed at risk of a number of heritable diseases that can be passed down from generation to generation.
In this blog post we will discuss the risks associated with line breeding racehorses and why breeders need to be aware of the potential health issues their horses may face.
Too close for comfort
While inbreeding makes it easier to ensure certain characteristics are passed on, it also makes it more likely a foal will inherit negative characteristics from both parents. Within a closed stud book, this practice speeds up the loss of genetic diversity of the entire breeding population which also can quicken the spread of congenital disease.
By selectively breeding a large proportion of racehorses with traits that are desirable (like precocity and speed) we have put them at risk of a number of diseases and genetic conditions.
The Good, the Bad, and the Ugly
The thoroughbred as a breed has a closed studbook, and all registered horses can be traced back to just three foundation sires from the 1600’s and 1700s.
Eight percent of all thoroughbred genes are derived from just 30 foundation mares while 95% of paternal genes can be attributed to just one stallion (the Darley Arabian).
Artificial insemination is not permitted in thoroughbreds, so breeding practices also contribute to the singularity of the breed.
Unfortunately, due to a narrowing population of broodmares and the fact that there are only a small selection of ‘popular’ stallions (all pretty much owned by just two stud operations) the global gene pool of thoroughbreds is smaller than ever before.
Thoroughbreds are now affected by several heritable muscle diseases caused by selective breeding for preferable muscle phenotypes.
A recent study on tying-up syndrome in thoroughbred racehorses found a correlation between increased inbreeding and the condition. They are also prone to other debilitating conditions which are thought to have a genetic basis.
Understanding and analysing coefficients
Not knowing which horses are carriers of known heritable diseases dramatically increases the risk of breeding an unhealthy foal. This is because, in a closed stud book, every generation loses genetic diversity, and with it, the mutations for carrying disease increase. Genetic coefficient of inbreeding is a more accurate method for measuring inbreeding in horses than paper pedigree evaluation.
Unlike pedigree-based calculations, a whole genome test can evaluate the actual pieces of DNA in each horse to accurately identify the proportion of inbreeding and make informed breeding decisions. Genetic testing can also identify alleles responsible for heritable disease and accurately predict whether a horse used for breeding will produce affected foals.
A horse that turns out to be a carrier of a genetic mutation does not need to be cut from the breeding population – it can still be used for breeding with an unaffected horse, though breeders should realize this practice will result in the gene being maintained within the breeding population, potentially causing diseased foals to be born down the line.
Whole Genome Testing with Victory Genomics
Whole genome testing is an invaluable tool for breeders who are looking to understand the genetic makeup of their racehorses and help reduce the prevalence of heritable diseases among broodmares and stallions.