Hunting Hard

*The green astrisk next to a mares name defines the mare as a "blue hen" mare:

 A “Blue Hen” is mare the foundation, “matriarch” of a prolific producing family. Always on the “dam side” bottom line of a multiple generation pedigree page This being through her daughters, granddaughter, etc. Generally producing multiple stakes horses of the highest caliber. Who then go on to do the same. Or dams of the same family that are unraced or lightly raced but produce stakes winners also.
They are far and few between...

 {Research indicates that the female X chromosome is responsible for the large hearts found in outstanding racehorses}

 Large hearts in the Thoroughbred Race Horse, (X and Y) Chromosomes and other definitions:

 

     Today, the normal weight of a horse’s heart is 8.5 pounds. Even though Secretariat’s heart was not weighed at autopsy, Dr. Thomas Swerczek, head pathologist at the University of Kentucky, estimated it at 22 pounds after finding the second-largest heart in Sham (Secretariat’s Triple Crown rival) and weighing it at 18 pounds.

"I have done thousands of autopsies, and I had noticed differences in heart size in horses before we did Secretariat," Swerczek said. "I had picked up the difference in the male and female hearts and noticed that some were bigger than others.

"But I didn’t pay much attention until Secretariat came along. He was completely out of everybody else’s league. Looking back at what he had done, it was easy to put a connection to it. The heart was what made him able to do what he did. It explained how he was able to do what he did in the Belmont Stakes – a mile and a half race (Secretariat won by 31 lengths in track-record time). You would have to have a large heart to do what he did. It would be impossible for a horse with a small heart to do that."

 

     Genetically, there are two sex-linked chromosomes in each horse. Males have an X chromosome, which they receive from their dams and pass to their daughters, and a Y chromosome, which they inherit from their sires and pass to their sons.  (Males have an X and a Y) The Y chromosome is much smaller than the X chromosome and carries less inheritable material. It is basically a sex determinant and when thrown into the mix at the moment of creation makes that individual a male.

If the sire contributes an X chromosome, then the resulting foal is a female.Because the large heart characteristic is on the X chromosome, large-hearted stallions that carry only one X chromosome, which they receive from their dam, can only pass on the large-hearted X chromosome. In the case of their daughters, they carry their sire’s X chromosome and one of the two X chromosomes carried by their dam. Whichever X chromosome is dominant is the one that is expressed. If a mare is a single copy (meaning she carries the large heart on only one of her X chromosome), she could pass on either the large-hearted X chromosome or the small-hearted X chromosome. Depending upon whichever X chromosome is dominant, that mare might express either a large heart or a small heart.

 

X Factor – Term that geneticists use to describe the female X chromosome, which is attributed with creating the large hearts found in outstanding racehorses.

 

Heart score – Term coined by Australian researcher Dr. James Steel to communicate his findings of the heart size on the electrocardiogram, which was gained by a correlation of heart weight, stroke volume, cardiac output and aerobic power.

Double copy – Describes the ideal broodmare, which has the large heart on both X chromosomes. If the mare is bred to a large-hearted sire, she always will produce large-hearted foals. Her daughters also will be double copy mares. Double copy mares frequently produce all winners. One double copy mare is the Thoroughbred Weekend Surprise, a daughter of Secretariat that is out of a double copy dam. Weekend Surprise’s dam, Lassie Dear, produced all winners and so has her daughter, which produced Horse of the Year A.P. Indy and millionaire Summer Squall. Both sires now are producing outstanding daughters, and when mated with large-hearted mares, are producing outstanding sons.

 

Single copy – Describes a broodmare that carries a large heart on only one of her X chromosomes.

A famous single copy mare that expressed the large heart, but carried the small heart of her sire on her other X chromosome is Kentucky Derby winner Winning Colors. She was measured and found to have the large heart coming from her maternal granddam, Miss Carmie, which carried one of the four largest heart lines from the famous broodmare sire Blue Larkspur. Winning Colors’ sire, Caro, was found at autopsy to have a small heart.

Breeding Winning Colors to a sire with a normal heart size would increase the possibility of her producing a foal with a small heart.

(http://horsesonly.com/crossroads/xfactor/heart-1.htm)