KHP is listing their new "rare" white TB - Page 2 - The Horse Forum
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post #11 of 33 Old 07-27-2011, 08:50 PM
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Thank goodness my white horse was a grey!!

he was homozygous for grey so ALL of his babies were all shades of grey.
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post #12 of 33 Old 07-27-2011, 09:00 PM
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That's the sad and unfortunate thing about grays, though, too--the melanoma... :(
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post #13 of 33 Old 07-27-2011, 09:07 PM
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seems wierd that the color would be an issue... When you see homozygous for black it means you'll have the black gene right? And no health issues... White is the only color associated with issues?

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post #14 of 33 Old 07-27-2011, 09:15 PM
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The KIT gene, which is where DW is located, also has some sort of function in the development of the gastrointestinal tract (keep in mind that many genes have multiple functions, and it's way more complex than how we always oversimplify genetics). dominant white in horses
This is why homozygous DW is lethal.

Although frame overo is on a different gene, it is still associated with incomplete development of the gastrointestinal tract.
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post #15 of 33 Old 07-27-2011, 09:21 PM Thread Starter
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I'm glad I started this I am loving all of this info.

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post #16 of 33 Old 07-27-2011, 09:22 PM
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On the subject of grays, I wrote this with another poster from a different horse forum--I've done a lot of research on the subject. The mechanism, as I hypothesize, has to do with the syntaxin intron's (the location of the gray allele) regulatory properties:

While Pielberg et al. suggest a likely mechanism for the whitening of the coat and development of melanoma in gray gene-carrying horses, the hypothesis is far from a complete explanation and does not explain the process fully. The researchers propose that the mutation leads to a proliferation of melanocytes and thus a predisposition toward dermal melanoma. This proliferation early in life, however, depletes the lifetime supply of the stem cells necessary for hair follicle pigmentation. This is why gray horses are initially born a darker shade of their adult base color, and this also explains why the coat turns white within a few years. No causative mechanism linking genotype to phenotype was proposed, however. Perhaps the duplication in STX17 leads to overexpression of NR4A3 not just in tumor cells, but throughout the body (or at least the melanocytes) of the animal. The increase in this gene could interfere with the cell cycle of melanocytes, thus leading to the cascade effect explained above.

Gray Horse Melanoma

Though the complete details of the process are not fully understood, the gray gene in horses causes a proliferation of melanocytes (skin and hair follicle cells) which eventually deplete and cause the underproduction of pigment in the hair. This causes horses to go prematurely gray in color and leads to an increased risk of melanoma (tumors). Tumors most often occur around the anus and on the tail. Internal tumors can also occur and are more dangerous. Tumors around the head and throat are often more deadly and aggressive in formation. Most tumors are often considered cosmetic blemishes. The melanomas may metastisize to vital areas of the horse’s body and cause death. Though this is not common it may still occur.

Potentially yes. Gray horses who have severe melanoma may lose function and die if it spreads into vital areas.

Gray is a simple dominant trait. Though not all horses who have gray will eventually develop melanoma. Homozygous grays are at higher risk for melanoma formation than heterozygous grays. Horses lacking functional agouti (the genes that cause black horses to be bay/brown) are at a higher risk for melanoma formation in general regardless of color. Therefore grays who have some form of functional agouti (regardless of underlying color) have a decreased risk of melanoma compared to those who lack it.

Affected breeds:
Any breeds or individuals who possess gray have the potential for Gray Horse Melanoma. (Mules/Hinnies can also inherit gray though melanoma has not been studied in them)

Approximately 10% of all horses have the gray gene. Approximately >75-80% of gray horses over the age of 15 will have developed melanoma caused by the presence of gray.

Available Tests:
There is a direct test for the gray allele, but this will not detect if a horse has melanoma. Only veterinary examination will determine its presence after a horse has already developed it. Research is being done to identify factors that reduce the risk of melanoma formation.

Most gray horses with melanoma can live normal lives without human intervention to attempt removal of the tumors. Tumors that ulcerate are often treated with freeze therapy. Tumors are often only removed if they interfere with the horse’s normal function. Consult a vet.

There is no way to proactively prevent melanoma in a gray horse. Not breeding for gray is the only form of preventing a foal which may develop the disorder. Since melanoma affected horses often live normal productive lives, most breeders do not proactively avoid using grays.

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post #17 of 33 Old 07-27-2011, 09:38 PM
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^^And that is exactly why Dobe will likely be the last gray horse I ever own. I had 2 melanomas taken off him a couple of years ago and now there are 4 more that need to go (one of which is right in the middle of his back where the bars of my saddle sit). He's only 9 years old so I am hoping, but not counting, on having him until his elder years.

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post #18 of 33 Old 07-27-2011, 09:43 PM
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But with that being said, it is also important what is not said. Melanoma in greys is far less serious than in other horses because mealanoma rarely metastisize in greys...they have a natural defense against it. In those instances when it does metastisize, of course it is very serious, just as with any metastatic cancer. Because grey horses have a natural defense against melanoma, they are and have been under study for potential human applications.

I mention this only because some people are hesitant to have grey horses due to potential melanoma, when in fact non-grey mostly white horses, particularly those with pink skin are in far more danger from lethal metastatic melanoma, even though the incidence of non metastatic melanoma may be higher in greys.

If that conflicts with your research, Bubba, let me know...
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post #19 of 33 Old 07-27-2011, 11:25 PM
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Then what is the White Fox. I worked on the farm where he was stood. He was a gorgeous individual and a camera hog.
Patchen Wilkes' white Thoroughbreds - The White Fox, Patchen Beauty, Precious Beauty & Late 'n White - Images | Barbara D. Livingston

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post #20 of 33 Old 07-27-2011, 11:38 PM
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Pretty horses but the first thought I had was 'Gah, what a PITA to keep clean!'

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