Heart Size in Pedigrees

by R. Soutar

The typical Thoroughbred horse has a heart that weighs around seven pounds. However, some exceptional runners have been found to have hearts twice that size and even larger. The immortal Secretariat had a massive heart weighing in close to twenty-five pounds! A large heart is a better pump and is certainly a factor in athletic superiority. There are many other factors that define the equine athlete, but heart size is one of the most important.

This topic has long been known to be important among the elite breeders and recently has received notoriety through Marianna Haunís book, "The X Factor". In her book, Marianna makes a convincing argument that the attribute of a large heart is passed on through the X chromosome. This is what she calls "The X Factor". Her book is highly recommended reading for all breeders of race horses.

 The idea that the large heart is passed through the X chromosome leads one to several interesting conclusions. First of all, a brief discussion of genetics is in order to help explain the theory. Every horse, like most creatures on earth, has 2 chromosomes in itís genetic makeup, that determine every aspect of itís being. Females have 2 X chromosomes, and males have one Y and one X chromosome. The chromosomes are a genetic road map that exactly describes every attribute. The X chromosome is larger and therefore has more space for the genetic markers that describe traits. Many traits are a function of markers from both chromosomes. Heart size, however, based on research carried out by Marianna, and others, seems to be carried on the X chromosome alone. Thus a male horse has a single marker for heart size, while a female has two. A male offspring inherits the Y chromosome from itís male parent, and one of two possible X chromosomes from itís female parent. A female offspring inherits the X chromosome from both parents. So a male horse has one chance of a large heart. He inherits one of two chromosomes from his dam, and that determines his heart size. A female horse has 2 X chromosomes and can have one, both, or neither that carry the large heart. For the case of a female, one X chromosome will generally be dominant, so it is possible for the trait to be carried, but not exhibited. Marianna coined the term "double copy" to describe a mare that carries 2 X chromosomes with markers for the large heart. These are the mares who are most likely to have exceptional produce records. These are the ones we want in our brood mare band.

The conclusion about heart size being determined by the X chromosome alone explains the pattern of broodmare sires. These are sires who are not, in general a huge success as studs directly, but whose daughters end up superior to the brood mare population in general. All the sireís daughters inherit his X chromosome, and if it carries the large heart, all of them will have at least one, and some of them 2 X chromosomes, that pass on the large heart trait. Thus although he cannot influence his sonís heart size, his daughters rate above average. Their offspring will benefit, and thus the sire gains a reputation as a broodmare sire.

When looking at pedigrees, it is useful to determine the "heart-line" in the pedigree, to look for evidence that the horse in question has a large heart. Since the trait is carried on the X chromosome, the heart-line is traced from sire to daughter, and Dam to son or daughter. If a sire is known to have a large heart, then he definitely passes it to his daughters, but definitely does not pass it to his sons. If a Dam is known to be a double copy mare, she definitely passes the trait to all her offspring, both sons and daughters. A single copy mare has a 50% chance of passing the trait to any given son or daughter. Tracing a heart line requires some knowledge of which horses in pedigrees carry the trait. The book "The X Factor" lists many known large hearted horses, common in pedigrees. This fact alone makes the book worth picking up.

Without direct knowledge of heart size, it is still possible to make a good guess based on highly correlated information. If a sire appears high on the brood mare sire list, it is highly likely he carries the large heart. Also, mares listed as "Reine-De-Course" are also likely to carry the trait. There are exceptions to both these rules, the most notable being Miss Disco, and her son Bold Ruler. Both were known to have small hearts. Other circumstantial evidence of the large heart is the ability to win races at more than a mile. The evidence is better as the race is longer and tougher. So graded stakes winners at classic distances are a good bet. There is one more factor that is useful as an indicator for broodmares. The produce record of a single or double copy mare should be considerably above average. Plus her produce should have a tendency to go a distance. Her sons are the best indicator, since they can only inherit the large heart from her. Using the above evidence and known large hearted horses can allow one to have a pretty good idea of the odds that a given horse carries a large heart. You can also have a vet use ultrasound or electrocardiography techniques to directly measure the heart size.

Heart size is just one factor that is important for race horse performance. It is however quantifiable and somewhat predictable. Therefore it is a useful factor for prediction of racing and breeding success.

To purchase "The X Factor" go to our book store!   Thanks!

 

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Last modified: March 07, 2010

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