Double Merle Dogs

[jackrussellgirl]

I was wondering if the Lethal Guinea pigs were the same gene as the merle colour in dogs? I found some really good pics of double merle dogs. For those who don't know... the merle colour is the freckled colour in some breeds like the australian shephard. They usually come in blue or red. Here are some pics...
Blue Merles
Red Merles
If 2 merles are bred together some of the puppies will have a double dose of the merle gene. These are called double merle dogs. They are usually deaf and have eye problems. The pupils are mishaped and sometimes they are born with no eyes.
Here is a link to pics of dogs with the double merle gene.
Double Merle Pictures - The White Aussies Project, at Lethal Whites .Com
Here are a few links to some eye problems.
Double Merle Eyes - The White Aussies Project, at Lethal Whites .Com
Aussie Eye Defects
The same gene is in horses and is associated with the palamino and paint overo genes.

[spoonyspork]

I'm no geneticist, but I know the cremello gene has nothing to do with Overos. Two cream-gene horses will NEVER produce a lethal white or horse with deafness or blindness, unless they have other problems (or both have the lethal overo gene). It's a common misconception that a 'white' horse born of two cream-gene horses have problems - they don't. They're just like any 'normal' horse, except with a bit more sensitivity to sunlight.

Overo will only produce lethal whites IF they have the lethal gene. Back when I actually bred my horse, I had a scare when one of his get came out with the overo paint pattern, and had him tested for the lethal gene. He didn't. I did LOTS of studying while waiting for the results though (he was a palomino with lots of white, but wasn't paint), and found out the horse can even look 100% solid and still carry the lethal gene. I also did lots more digging into finding pictures of his ancestors, and one of the first 'quarter horse' parts of his bloodlines, some-odd hundred years ago, turned out to be an overo paint. Another score for bad idea to breed, any time. I could trace my guy back to the Darly Arabian, and there was still stuff in there that could have been important to the life of future generations that I hadn't known about.

As for whether it's related or not... I have no idea. Somewhere in the lethal whites discussion thread here, I asked a similar question about overo horses. I got a nice explanation PM'd to me, but I deleted it a few weeks back (it'd been over a year since I got the PM). I THINK the person said the genes that make overo and the genes that make dalmations and roans are completely different. The merle pattern looks REALLY close to what you see in guinea pigs, though... so maybe!

I think the person who PM'd me might've even been one of the mods... or at least someone who's still around. Maybe they'll step in and answer

[salana]

I believe, based on similarities to known genetic problems in mice and hamster, that the gene involved in lethal white guinea pigs is called Mitf. Mitf has a lot of effects on color, eye and ear development, and bone and tooth development.

The merle gene in dogs is called SILV or Pmel17. SILV "tells" other genes to start working, and one of the genes it "sets off" is Mitf. So, a double merle dog might have a perfectly fine Mitf, but if it has a broken SILV that never talks to Mitf, its Mitf never does anything (or much less than normal) and it comes out looking like it has a broken Mitf.

Overo lethal white in horses (and megacolon in rats) is one of a pair of genes, I don't remember which of the pair it is. The pair consists of Endothelin-3 and Endothelin Receptor B. EDN3 is like a key and EDNRB is like a lock, so if either is broken, it doesn't work. The endothelin pair is involved in nerves and pigment cells moving from an embryo's spinal cord, all across its body. If they don't work, nerves don't go to the intestines and color doesn't go to the skin, so you have a white horse or rat who can't move their guts to poop, so they die from that.

There are Mitf problems in humans, (Waardenburg Syndrome type II, Tietz Syndrome) but they're relatively minor, with none of the bone problems, just deafness and pigment problems. Endothelin problems are a bit more serious in humans, where they're called Waardenburg Syndrome type IV, and the megacolon (lack of intestinal nerves) is called Hirschsprung's Disease. It is possible to maintain humans and rats with megacolon, sometimes, with enemas and a special diet. I have never heard of it being done with horses.

Horses have a roan coloration, and it's believed that breeding roan to roan occasionally produces foals that are miscarried at an early stage, but this is not entirely agreed upon among horse people.

I hope this helped clear things up!

[spoonyspork]

Thanks, Salana! That really cleared things up

I just have one more thing to add - this link will take you to the wikipedia article on lethal white in horses (there also discounts the myth of double-dilute horses being lethals).

I also think there needs to be a lethal white guinea pig entry on wikipedia. Maybe I'll start one one of these days, and let those more knowledgeable than myself contribute to it as they see fit. It'd be useful for people getting the idea that it's a bad idea to breed dalmatians and roans and the like.