There is a lot of merit in what Owner-Breeder terms "the Rasmussen factor". This is a theory of "inbreeding to superior female families through different individuals". The inbreeding to qualify must occur in the first five generations and be carried in both sides of the pedigree. This is a theory I find most agreeable and worthy of study.

Leon Rasmussen wrote a series of articles that were reprinted in the Australasian Bloodhorse Review between August 1996 and January 1997. The original articles having been published in the American journal Owner-Breeder between July 1993 and March 1994. These articles are recommended reading. A new book has recently been published and if you do not have access to these articles I recommend you read this publication.

The sire Kalamoun is an example of a Rasmussen factor horse given in one of the articles. There are many other examples of this pattern appearing in successful sires and mares.

A possible reason for this being successful is that it is believed genetic material that sits closely together on the chromosomes is less likely to be separated when recombination of this material takes place. It is possible that certain stallions and mares that appear to be particularly prepotent have in their chromosomes important genetic markers that sit closely together and these are not separated when recombination takes place and don’t crossover, remaining fixed on their chromosomes and this has a favourable results. That is, not all genes obey the law of independent assortment. This may explain why the Rasmussen factor or for that matter any other preferred pattern appears successful.

X Factoring the Rasmussen Factor

I prefer and use a variation of the Rasmussen factor. The step I take and that I believe is worthwhile exploring is "inbreeding to superior female families through different individuals SEX LINKED TO THE X CHROMOSOME".  If possible I would like to do this through full sisters on the X trail and preferably through a son of one and a daughter of the other. Backline is one horse I have bred that is an example of this type of mating. If this is not possible the next best option is just to get a different strain of the same female family on the X trail. I apply this pattern even though I know that with a son the stallion will not pass on this strain of the X but for still consider this as fitting my preferred pattern. With my pattern all that is necessary is that in theory if a filly results a similar X chromosome can be inherited from both the stallion and mare being mated.

The merit of using different individuals is that it provides genetic variation as each parent has its own unique genetic makeup. I believe Leo Rasmussen has made a valid observation that it is desirable to breed to superior female families through different individuals. It obviously multiples the chances of inheriting the genes that make a particular family successful and at the same time reduces the risk of inheriting genetic defects.

The reason I would extend the Rasmussen factor to include "sex linked to the X chromosome " is based on the premise that if important genes are carried on the X chromosome then this is the only way they can be inherited. It is only through the daughters or son's daughters that this genetic material is transmitted. One other possible advantage sex linking this theory is one could use individuals a lot further back in the pedigree chart. Not in the literal sense as obviously you can only mate the immediate parents.