On the Existence of the X-Factor

[Blenheim]

No doubt this subject been discussed countless times in this and many other forums.

I've been doing some research on a few Derby contenders, some having the X-Factor; I’ve always wanted to know definitively whether the X-Factor exists or does not exist. Attached please the link to an interesting and informative discussion by some sharp scientific minds that really know their genetic stuff. The heart of the discussion begins on page 8, the prior pages set the stage.

The question of the existence of the X-Factor is answered.

Here is the link: The X-Factor

[bobphilo]

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.

[bobphilo]

Just to add, a mare will pass down her mitochondrial DNA to both her male and female offspring but only her female offspring will pass down this mDNA to their offspring.

[Katewerk]

I think the reference here is to the x-chromosome, passed down by the sire, but inherited from his dam.

[bobphilo]

Originally Posted by Katewerk

I think the reference here is to the x-chromosome, passed down by the sire, but inherited from his dam.

I think you're right. I think we were talking about 2 different types of inheritance. I thought the X factor had to do with mDNA but it makes sense that it refers to the X chromosome. What they have in common is the genetic influence of the dam. Thanks for the clarification.