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Dwarf Miniature Horses

Dwarfism in the Miniature Horse
By Janell Jensen

In order to lessen the chances of producing dwarves, or eradicate the risk all together, we need to learn all we can about them as well as find a way to test for the gene that causes it. Since it is such a prevalent problem in our breed, I feel that sharing what we know and learn is the first major step in this process, which can only be good for the breed as a whole. I also believe that all dwarves should be registered (as dwarves)so that there can be a record of their births. I think that is the only way we are going to be able to lessen the chances of producing a dwarf. We need to be more selective in our breeding as well, by not breeding any horses that have any dwarf characteristics at all, and seriously thinking of gelding a stallion that has produced a dwarf foal, and taking a mare out of you breeding program is she has produced a dwarf foal. By taking the mare out of your breeding program, I mean that she should not be sold to someone else for their breeding program either, only as a pet. My goal with this paper is to educate the general public as well as the Miniature Horse industry as a whole.

I am unaware of any actual scientific studies to prove or disprove the existence of a dwarf gene and how it is passed on. Nevertheless, it used to be believed that only the stallion carried the dwarf gene, but now it is believed that both the stallion and the mare can carry the dwarf gene.The dwarf gene is a recessive gene and it takes 2 parents having the recessive gene to produce a dwarf foal. So once a mare & a stallion have produced a dwarf foal you KNOW that they both have to be carriers of the recessive gene. Breeding these two horses together again will give you a 75% chance of them either producing a dwarf foal or producing a normal foal that is carring the recessive gene. NOT GOOD! There is a 25% chance that these two horses will have a dwarf foal, a 50% chance that they will produce a normal foal that is a carrier of the dwarf gene and only a 25% chance that they will produce a foal without the recessive gene..... NOT GOOD ODDS EITHER! So anyone who breeds known dwarf producing horses only have a 25% chance of producing a foal that is not a dwarf or a foal who is not a carrier of the dwarf gene... DEFINETLY NOT GOOD ODDS, if ya ask me!!!! I also believe that there are MANY more dwarves being born each year than most people want to believe or will admit to.

Best case scenario (if the unknown horse is not a carrier): 50% chance of
producing a carrier, 50% chance of producing a normal horse (not a carrier).

If the unknown horse is a carrier as well: 25% chance of producing a dwarf,
50% chance of producing a carrier, and 25% chance of producing a completely
normal horse (not a dwarf and not carriying the dwarf gene)

The life expectancy of a dwarf is not nearly as long as a normal miniature horse. It depends on the severity of the horse’s dwarf characteristics. These special little horses are never with us nearly long enough! Yes, a horse CAN be a little dwarfy, unlike being a little pregnant! There are many degrees of dwarfism and many different types of dwarves. Dwarf characteristics will be passed on through the genetics into their offspring, whether the genes manifest themselves in this generation or not, they now reside in the horse’s DNA and will crop up, be it now or later. What is fascinating is that coiled inside an animal’s DNA are secrets about its heritage as well as its progeny.

In the August/September 1998 issue of Miniature Horse World, dwarfism is described as the following:

Dwarfism n. — underdevelopment of the body, characterized primarily by abnormally short stature, often with underdeveloped limbs and other defects. Causes include genetic defects, pituitary or thyroid malfunctioning, kidney disease and certain other disorders. (Rothenburg and Chapman 1989)

The structural anomalies of dwarfism in horses are essentially the same as they are in humans. The most common type of dwarfism is the brachycephalic dwarf (brachy = shortness, cephal = an association with the head), whose middle third portion of the head, namely the nasal bridge, is characteristically low and flat. In addition, characteristic of the Brachycephalic dwarf are enlarged joints, a short neck, and excessively short limbs.

Achondroplasia, on the other hand, is an inherited disorder in which a defect in cartilage and bone formation results in a form of dwarfism characterized by short limbs on a normal trunk. (Rothenburg and Chapman 1989)

Some horses show vague characteristics, which are difficult to discern. Some show dwarf characteristics and are not full dwarfs; they can live a relatively full life, but often begin to suffer from some type of joint degeneration after reaching maturity. Others begin their lives looking almost completely normal and begin to change as they mature. Still others have such severe manifestations that the animal has to be euthanized.

Some common phenotypic characteristics of Dwarfism in the Miniature Horse Breed:

1. Achondroplasia—(legs do not grow in length). Normal bone growth does not occur and often develops unevenly at the joints, causing crooked legs.

2. Dwarf foals are sometimes born with contracted tendons or tendon laxity. Joint enlargements and joint deviations are common, often becoming progressively serious with age. Extreme cow hocks, extremely short gaskins and sever sickle hocks all with varying degree of joint laxity and /or joint weakness are also common. Premature Arthritis is common as well.

3. Some types of dwarfs have an over bite, or an under bite. If an undershot jaw is present, the molars may also be out of alignment, requiring that the teeth be floated more frequently than for a horse with a normal mouth. Highly placed nostrils always accompany the undershot jaw.

4. Brachycephalic dwarfs have a large buldging forehead with an extreme dished face, overly large eyes (sometimes placed at uneven angles) and nostrils placed too high up on the face. A second type of dwarf has a more normally shaped head and eye and longer neck, but its head and body are still oversized when compared to the length of its legs. This second type of dwarf does not usually have an undershot jaw.

5. Head obviously longer than the neck. In some dystrophic dwarfs, the neck appears to come directly out of the shoulders.

6. Girth depth is greater than the leg length. Disproportionately oversized entrails and genitals. That is why so many dwarfs have the big “pot bellies.”

7. Vertebra deviations are common.

8. Often unable to rear or stand on hind legs. Some types have an odd “tilting backward” gait with shoulder higher than the croup.

9. Sometimes associated with the various dwarfism syndromes are less obvious characteristics such as mental retardation, heart and other internal organ defects, sterility, shortened life span, arthritis and inactivity or depression (both of which are probably due to pain).

There are over 200 variations of dwarfism characteristics that have been cataloged and well described in humans alone. In the dwarfed miniature horse, most of these characteristics are recessively inherited (both the sire and the dam appear to have normal conformation).

To read more of Janell Jensen's article please got to page 2

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