It still amazes me that in this day and age of "designer dogs" and even the popular crossing of domestic and wild cats (not to mention all the livestock crosses), that when you mention a crossbred horse people tilt their head, cock an eyebrow and a sharp "huh" comes out through pursed lips.

A quick look at genetics tells us that each "trait" comes in pairs, one from the father and one from the mother.  When talking of purebreds, they share commonality in many many areas, thereby producing homozygous genes (same), thereby limiting the variation of production.  While this certainly can lead to concreting in some traits that are desirable, this also means opening up the door to solidifying traits that are not so desirable.

Take for example, the Quarter Horse.  This modern breed was developed from many many breeds, including Thoroughbreds, Mustangs and even Draft horses.  What started out as a haven for crossbreds and producing superior, hearty individuals for ranch work and racing, has become a nesting ground for all sorts of genetically linked diseases such as HYPP, Herda and navicular.  Why? 

Sooner or later, people began to fancy one horse more over another, and began heavy breeding to this individual.  Line breeding and even inbreeding became "okay" and it was common to see lineages dotted with the same horse over and over.  While these methods do have a purpose in creating homozygous traits for the positive, they also create the meeting of many recessive genes that do not produce such popular results.  The odds are greatly increased for genetic defects when this happens.

Commonly, inbreeding reduces fertility, overall health and mental stability. Inbred animals are more prone to diseases such as infections and cancer, and more likely to be "highly strung" (or even ill-tempered) and difficult to deal with.  Immune deficiencies are created, along with digestive issues and defects in the nervous system.

If inbreeding increases homozygosity (same), crossbreeding maximises heterozygosity (different). Crossbreeding takess advantage of a phenomenon which is widely talked about but poorly understood: heterosis or hybrid vigour. This term is used to describe the burst of fertility, good health and growth that is seen in the progeny when two unrelated breeds are mated. The longer that these breeds have been separated, and the greater the differences between them, the stronger will be the resulting hybrid vigour.

On another note, some genes that have not seen eachother in many generations, also come together to create a super burst of hybrid vigour; often producing a super superior animal that is very hard pressed to reproduce.  This is the basis for many such sires that are repeatedly bred, but never duplicate their super traits.  You can see this in other species, such as "Hogzilla", "Super Cow" and most recently discovered, huge, aggresive crossbred bears.

Hybrid vigour, when done correctly, will produce not only a larger, healthier animal, but also more mentally stable and have a higher rate of trainability.  Stronger immune systems, effecient digestive systems, higher intellegence and increased athletic ability are all benefits reaped when crossbreeding is done successfully.  Obviously, crossbreeding can have negative results when inferior animals are used, just as in purebreds.

With my husband being a trainer and clinician for almost 30 years now, we have literally dealt with close to 5,000 horses from every type and breed.  We began to notice that quite consistantly, the crossbred horses that came in for training seemed to overcome their issues quicker and retain more of their training than many purebreds.  They had much less health issues, better feet and could adapt to different situations easier.

We found that the crossbred horses were much better matched for their owners than many of the purebreds.  They seemed to be more forgiving and tolerant overall, while their purebred conterparts tended to be emotional or even lacking emotion and participation in a partnership.

For many owners, I belive that crossbred horses have alot of offer in the way of better health, mental stability, trainability and versatility.