TRAINING & BREEDING QUALITY ROCKY MOUNTAIN HORSES
Note: Sometimes it is difficult to determine the status of the eye just from a genetic test and determine whether a horse has any genetic abnormalities. As seen above, sometimes the eye can look abnormal and "buggy". This is also not an exact science. A veterinary optometrist/ surgeon or eye specialist should be able to examine the eye and determine what structures are abnormal. If there is any question seek veterinary assistance.
Abnormalities also do not prohibit the horse from experiencing a riding career. Many affected horses go on to ride down the trail. However, an owner must note that symptoms MAY OR MAY NOT progress as a horse ages (considering there could be other underlying conditions- i.e. Uvetis or injury). Knowing your horses exact conditions will assist you in your relationship with an affected animal.
There is not a genetic test for MCOA gene however we can test for silver genes.
The best guidance I have regarding buying, owning or riding a ZZ/double silver horse is to know them well. Sometimes having a foundation of training from a professional trainer who understands the genotype is your best bet.
Next, understand the underlying physical limitations they may have. Knowing how their vision changes can drastically help you ride and understand them.
Also note that if you experience a certain reaction, they may be experiencing changes for one reason or another- Always seek veterinary assistance!
Below are excerpts from Bonnie Hodge's book Rocky Mountain Horses- where she takes an in depth scientific approach to explaining and proposing solutions:
Each side of the square represents the two genes 1 horse could produce in a single silver carrier. Inside the square represents possible genetic make up of offspring. Each possibility has a likelihood of 25% when two single-silvers are crossed. When using a ZZ individual you are 50% -100% likely to create double silver offspring.
Terms:
Silver Dapple or Chocolate- Refers to the phenotype of the horse or the silver dapple gene
Genotype- genetic make up; consists of two genes or alleles that can be passed to offspring.
Phenotype- what the animal physically displays i.e. color
Carrier- a horse can carry a gene to pass to offspring but it may not exhibit it in it's phenotype.
Click to enlarge
Blond Brownie is an example of a double silver horse with a nice foundation. He is completely ridable and trusts me to guide him from a light area to a dark indoor seen here.
MCOA
or Multiple Congenital Ocular Anomalies aka ASD (Anterior Segment Dysgenesis)
FUN FACT!
The Silver dilution exists in many breeds:
Rocky Mountain
Kentucky Mountain
Missouri Fox Trotter
Tennessee Walker
Shetland
American Quarter Horse
Icelandic Horse
It is theorized that the MCOA condition also exists in these other breeds. Considering that the expression of the phenotype is not as dramatic as it is with the Rocky Mountain or Kentucky Mountain, the breeding for ZZ horses is not as popular in the other breeds. There is a lack of scientific study for any other breed other than the Rocky. To fully understand the ZZ - MCOA link we must have more information and scientific evidence to understand how it is linked.
These two terms are synonymous and describe a unique set of eye phenotypes or conformations present at birth. The silver (chocolate) gene is associated eye problems and they seem to be inherited together but as two separate genes. Research still needs to be performed to accurately document the variations within the silver color and eye problems but it appears the Rocky breed isn't the only one. Shetlands, Miniature, Morgan, Halfinger, American Quarter horse, Missouri fox trotter, Icelandic, Kentucky and Mountain Pleasure breeds also carry a silver dilution. It is theorized this gene responsible did come from a very early ancestor common to most of these breeds.
The silver gene is responsible for the chocolate color most people associate with the Rocky Mountain horse. Many Rockies carry the silver dapple gene and this is very important to make sound breeding decisions to avoid inherited eye disorders. Horses can carry one or two copies of the gene. If they have one copy they are least likely to have the full syndrome of effects than horses with two copies of the gene. If a horse has two copies they may have one or many different physical changes to the eye. These can range from cataracts or cysts to a complete inability for the lens of the eye to function or detachment/separation of the retina.
A horse with two copies of the silver dapple gene are likely to have their vision impaired in some fashion, however a horse with one copy of the silver dapple gene may not physically show any eye abnormalities or may have cysts present (cysts won't cause visual limitation). Double Silver horses may not show it's owner any signs that limit his willingness to be ridden. In some cases the symptoms seem to become more apparent later in age. HOWEVER, there are many other environmental eye ailments that have wrongfully associated with the silver gene and named MCOA. (mainly Uvetis that is contracted environmentally)
Impacts on breeding decisions:
It is a popular to have brood stock that carries two copies of a color gene to pass on silver/chocolate color 100% of the time. This has been the case of chocolate colored Rocky Mountain horses for many many generations. In light of MCOA, sound breeding decisions are needed to avoid breeding silver dapple horses to silver dapple horses. Genetic testing can inform breeders how many silver genes are in their brood stock and help to make educated choices regarding the silver dapple gene. It is advisable to breed a chocolate/silver gene horse to any non-silver gene horse - such as a black. 100% of the offspring would have minimal chances of eye abnormalities, at least avoid the full MCOA syndrome that would impair their vision. A chocolate horse with one known black parent would only have one copy of the silver gene. A horse with both chocolate parents has at least 25% or higher chance having visual abnormalities. See the chart below to see the different combinations with the silver gene. Sometimes the silver gene is present but is not seen such as a sorrel. This is known as a carrier.
Punnett square 101
Carrier colors include: Sorrel, palomino, cremello, red dun, red roan, perlino, grey. These colors would typically hide the silver dilution. Other color modifiers may also hide the color dilution. As a rule of thumb, if the horse has a base of black or E_ and 2 silver (ZZ) it is usually lighter or "chocolatier" than normal BUT this isn't always true.
Links:
https://www.rmhorse.com/breed/genetics/