The ‘X Factor’ Theory: What is it?

The Races - Caitlin O'Sullivan
26 March 2020
Hearing a horse being referred to as having the ‘X Factor’ is nothing new. It’s a term used to describe someone special, a champion, a horse with a great heart - and in some cases, this is quite literally! 
Simply put, the ‘X Factor’ theory proposes an explanation for horses with abnormally large hearts. It is a hypothesis which states that some broodmares have a recessive gene which, when matched with a carrying stallion, will produce offspring with larger-than-average sized hearts - up to three times the size of a normal, 3.6kg thoroughbred heart. It was first discovered in a racehorse Eclipse and is found in four major thoroughbred lines - Princequillo, War Admiral, Blue Larkspur and Mahmoud. 
If you watched Secretariat’s 31 length 1973 Belmont Stakes victory, the fact his heart was three times the size of an average thoroughbred heart will come as no surprise to you. At 10kg, his heart left scientists baffled and provided some explanation for his supernatural performances. A heart this size meant the champion could run sectionals in the one and a half mile Belmont stakes progressively quicker without tiring, while his rivals did the exact opposite. 
Interestingly, those who carry the gene are noted as being excellent broodmare producers however generally failed to produce male offspring with any great ability. From this, it is theorised that the gene for large hearts is carried only on the X chromosome, meaning stallions with the trait can only pass it via their daughters. 
Disclaimer: If I’ve lost you here, you’ll want to skip the following paragraph. If not, get ready for a biology lesson to break things down further … 
Males have an X and a Y chromosome, while females have two X chromosomes. A foal receives two chromosomes - one of its dam’s two X chromosomes and either the X or Y chromosome from its sire. If the foal receives two X chromosomes, it will be a female; if it receives an X and Y chromosome, it will be a male. 
Simply put, X + X = Female while Y + X = Male. 
Therefore, a stallion with the X factor gene can only produce female foals with the gene, as the X chromosome needs to be passed, while a broodmare with the mutation can produce both sons or daughters that will be affected. 
Let’s use Secretariat as an example. The dam of Secretariat, Something Royal, was sired by Princequillo, one of racing’s modern-day carriers of the X-Factor gene. However, only his daughters that received the gene can pass it on to their offspring - as was the case with Secretariat. 
As time and science has developed, many more modern-day champions have been found to carry the gene. New Zealand bred legend Phar Lap’s heart was 6.35kg, while champion American thoroughbred Sham’s heart weighed 8.2kg. 
They say a horse gallops with his lungs but perseveres with his heart… And an extraordinary large heart makes this all the easier. 

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