Genetic mechanisms in equine dwarfism

Dwarfism is considered rare in large horses and considered fare more common in USA miniature horses, especially among those who breed horses with known dwarf ancestors.

Some suggest that equine incest may contribute to horse dwarfism and some breeders report dwarfism as "the result of an accidental breeding between a full brother & sister".

While the genetic mechanism remains unknown, it appears that USA miniature horse breeding programs vary widely in dwarfism rates and some suffer from very high reproductive mortality.

The link between genetics and miniature horse dwarfism is not surprising, given that the hallmark of dwarfism is a small size. 

Pedigree does not guarantee safe breeding. One author claims that "bug" (their dwarf horse) had a champion AMHA pedigree:

"This is the product of poor breeding management. This little gal has a tremendous pedigree with at least two AMHA national champions in her near background.  The gene that causes dwarfism can be found in every pedigree, pet quality, show quality, as well as the "taller" minis."

Genetics experts and dwarfism

So how do we explain the wide variation in dwarf production rates between miniature horse breeds and individual breeding programs?  There is what some experts say:

J.C. Williams, owner of the Del Tera Miniature Horse Farm in Inmon South Carolina, claimed that breeders have a huge variation in death rates, with some of the irresponsible breeders killing up to one-third of their foals each year.
 

An article from Miniature Horse World Magazine (1998) confirms that human studies prove a genetic link to dwarfism, and the author speculates about the genetic dynamic in dwarf miniature horses:

In the case of dwarfism one of the two following systems could (possibly) be at work in its development in the Miniature horse.

1- Autosomal recessive trait--Both parents pass on a recessive gene and the trait is expressed in the offspring.

2-Autosomal dominant trait--One (or both) parents pass on a dominant gene and the trait shows up in the offspring. The trait would have to be expressed in one of the parents or could be [caused by a new mutation (Crandall and Crosson 1993)]

There are over 200 variations of dwarfism characteristics which have been cataloged and well described in humans alone. Most of the human dwarfism characteristics have been proven to be genetic in nature. In the dwarfed miniatures, most of these characteristics are recessively inherited (both the sire and the dam appeared to have normal conformation). Many of the scientifically identified characteristics have been researched individually (Bowling 1996).

The Greenberg Center for Skeletal Dysplasias:  conducted human dwarfism studies and notes that people with dwarf characteristics have a higher probability of producing dwarf offspring. 

“Achondroplasia is an autosomal dominant condition (see the genetics section for further details). This means that a person with achondroplasia a 1 in 2 or 50% chance of having children with achondroplasia.”

Andrea Rollins, published a column where she notes the correlation of genetics and equine dwarfism:

"Genetic soundness is also important in miniatures, as bad bites, dwarfism, and locking stifles are common.  Although some people feel that pedigree is not very important, and that judges never see a horse's papers... I feel pedigree is somewhat important to consider in a breeding animal because pedigree gives the breeder some knowledge of a horse's genetics and predictability."

Chris v d Merwe, writing in the "ask the expert" column of Horse & Stable Magazine, notes that the limited gene pool has fostered equine dwarfism.  He gives a strong condemnation to those who breed miniature horses with dwarf traits:

"Breeding miniatures is a bigger responsibility in South Africa than breeding most other horses because our gene pool is very small. It is cruel to just breed to have another little horse-like animal shoved into a body that can not move or work properly! (Sorry I am so straight forward, but this is a serious situation today in the breed.)".

Dr. Ron Friedman notes in an article published in Northwest Miniature Horse Club Newsletter, July-Aug 1996, Vol. 1, no. 67, that the smaller AMHA "A" type miniature horses have more dwarfism and foaling problems:

"Type A mares also have more dystocia and foals with congenital problems (abnormalities developed during the pregnancy) like dwarfism and jaw malalignment."

Andrea Rollins accuses breeders of the deliberate introduction of dwarfism into the American miniature horse in her publication titled Breed Standard:

"In the past century, dwarf horses were purposely used in breeding programs because of their diminutive size (even compared to the Miniature!).  Thus, the Dwarf gene is floating around in many bloodlines today, and it is difficult to locate as the gene has not been identified by geneticists."

Equine Dwarfism in foreign countries

Some cite evidence that miniature horse dwarfism and congenital defects are more prevalent in the USA miniature horse breeding programs than  in foreign countries. 

In England, many of the smaller Dartmoor Ponies are well-within the height requirements of the USA Miniature Horse Registries, yet dwarfism is virtually unknown in the breed.  According to the owner and breeder at the Miniature Pony Stud inside Dartmoor National Park, dwarfism is unknown in their breeding program.
 

The USA miniature horse breeders

Miniature horse breeders in the USA have mixed opinions about the causes and prevention of dwarfism.

John Eberth, AMHA Genetics Committee Chairman has published some information on American miniature horse dwarfism on behalf of AMHA. 

The text below suggests that the AMHA's does not want to stop dwarfism by culling all dwarf gene carriers (the only real solution to prevent future dwarf suffering), because it might hurt the profits of the miniature horse breeders:

"In my opinion, it would be very foolish of our breed and any Miniature horse registry to demand culling of all carriers, we should look at other breeds of animals and how they have dealt with genetic problems and how it was managed out, rather than go heck bent on instant cull and get it over with so to speak.

You could destroy more good than bad and more people's livelihoods than you realize."

Dr. Eberth provides his medical opinion on equine dwarfism.  Note:  It's not clear if Mr. Eberth is licensed to opine on equine genetics, as his medical qualifications do not appear on the AMHA web site. 

We need to understand that it is NOT polygenic. Polygenic means that there are mutiple (sic) genes involved in one disease or characteristic trait. What is thought is that there are multiple different dwarf diseases with each having a possible different gene involved for each different disease.

I have characterized POSSIBLY 4 different types with at least four different candidate human genes with homologous phenotypes as matches.

Now it is possible for one dwarf to have two differnet (sic) dwarf diseased genes and express both of them. This again is not polygenic but a horse with two different disease genes being expressed at once.

Also, I do not know of a human genetic dwarf disease that does not show signs until later as a result of older age, there are over 200 types of human dwarfisms though I will have to see. As for these seen in Minis, these are NOT due to age or environment as a sporadic result. They are due to genetic mutations that have been inherited over time thru generations.

Also, you do not get part of a dwarf gene and therefore are less of a dwarf. It is all there or not. And if you are a dwarf you have two copies of the recessive dwarf gene, not some of the gene but all of it. What does occur is Penetrance and Expressivity.

These genetic terms explain how dominant and recessive genes interact within an individual. What you might think is a minimal dwarf is actuallty (sic) a heterozygote mini just the dominant normal gene is being "overpowered" by the recessive dwarf gene."

Also, note this presentation on miniature horse dwarfism by the AMHA.

Minibreeders.com has published web pages with photographs of severe miniature horse dwarfism, and claims that miniature horse dwarfism cannot be prevented:

"Dwarfs ARE a REALITY of raising Miniature Horses".

Barbara Ashby in an article in Miniature Horse World Magazine suggests that the inherent breeding for small size has made miniature horse dwarfism impossible to prevent:

"Any breed of animals that attempts to reduce size has to accept dwarfism as the unwanted by-product.  We as breeders of miniature horses can do everything possible to minimize the possibility but it will always be with us".

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