Just what is the "large-heart" trait? It is the passing of the X-Chromosome of a large hearted stallion through his daughter to her son.
Through genetic research, it was discovered that the typical thoroughbred horse has a heart that weighs seven pounds and that heart size is inherited through the Dam's X-Chromosome. However, some exceptional runners have been found to have hearts that were two to three times the normal size. When the autopsy was performed on the immortal Secretariat, it was found that he had a massive heart weighing in at twenty-two pounds!
Traitt
A large heart is a better pump of oxygen to the blood and thus a major factor in athletic superiority and credited to Secretariat's 31 length romp in the 1973 Belmont States at 1 1/2 Mile.
Like most creatures (including humans) on earth, there are 2 chromosomes in a horse's genetic makeup, that determine every aspect of it's 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.
Heart size, based on genetic research carried out by noted researcher Marianna Haun, 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 it's male parent, and one of two X chromosomes from it's female parent.
A female offspring inherits the X chromosome from both parents. So a male horse has one chance of inheriting this trait. He inherits one of two chromosome from his dam and that is what determines his heart size! A female horse has 2 X chromosomes and can have one, both or none that carry the large heart trait. A female with 2 large-heart X chromosomes will always pass on the trait to her sons.
Have you ever wondered why a top performing horse does not sire colts with the same ability but does produce mares that are?
Top broodmare sires are able to pass the Large-heart X-Chromosome to his daughters who in turn can pass it on to their sons. All the sire's daughters inherit his X-Chromosome, if it carries the large-heart trait, all of them will have at least one, and some will have 2 X-Chromosomes that pass on the Large-heart trait.
Thus although he cannot influence his son's heart size, his daughters rate above average. Her offspring will benefit and thus the sire gains a reputation as a broodmare sire! When looking at the Kentucky Derby winners and tracing their broodmare sire lines back to the possible passing of this powerful X-Chromosome, a high percentage show War Admiral and Mahmoud pedigree lines.
Princequillo and Blue Larkspur are two other proven large-heart stallions.
So when handicapping the 2011 Kentucky Derby look for those runners that have the possibility of inheriting this special X-chromosome on his Dam's side.
Also keep in mind that the Large-heart trait shows up a high percentage of the time in the second place finisher. In 29 of the last 46 Kentucky Derby 1st and 2nd place runners had the X-Chromosome of the Large-heart trait!
Kentucky Derby Winners and the Large-Heart Pedigree Traitt
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