I had posted about this in a previous thread. While an individual gets equal amounts of the DNA in the cell nucleus from sire and dam. There is also DNA in the cell's mitochondria, the little organelles that produce energy in the cell. All of an individual's mitochondrial DNA comes from the dam and is passed down intact through every generation through the dam. This is likely what is called the X factor since this extra genetic contribution of the mare is not commonly known and under-rated compared to that of the stallion. An extreme example of this error is the dosage method which only considers stallions as chief-de-races in the pedigree.
As mitochondrial DNA is passed down intact through the generations from the female, unlike nuclear DNA which is recombined each generation, it is an excellent method for researching ancestry. Anthropologists have discovered that all human beings have the same mitochondrial DNA, except for mutations, tracing back to a single ancestral female known as Genetic Eve.
In terms of race horse breeding. Since the mitochondrial DNA, responsible for the all important cellular energy production, beginning from the 1st fertilized cell, comes entirely from the dam, she therefore has an eXtra influence on the offspring.
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