Category: Special Dog, Cat, and Pet Medical Conditions
Posted By: kim k Date: Sunday, 10 April 2005, at 10:10 a.m.
This is a copy of a post I worked really hard on for the Yahoo Deaf Dogs group and the Yahoo Lethal White Aussie Rescue group, to show how and why merle to merle breeding produces double merle pups and other breedings do not. I have added a 4th pairing, which shows why double merles are sometimes used in breeding to obtain 100% merle litters (the only way, statistically speaking, you can get all-merle litters, because breeding a merle to merle -- as you will see -- won't do this).
The Merle Gene.
The "merle" gene, while it determines the appearance of the dog, is totally distinct from the color gene. In Aussies, the "color" gene determines if the dog is going to be black or red --whether two colors (a bi - black and white or red and white) or three colors (a tri - black, tan and white, or mahogany, tan and white). Then there is the "merle" gene, which results in color dilution, and the typical dappled black or dappled red, that makes a blue merle or a red merle.
The merle gene is dominant over the solid gene. Let's call the merle (dominant) gene a capital M and the solid (recessive) gene a small m. There are only 3 combinations you can get out of these genes: MM, which is a double merle; Mm, which is a regular merle, and mm which is a solid.
Do you remember in high school the grids (squares) we used to make showing what color eyes the children of blue (bb) and brown (BB or Bb) eyed parents would have? The same thing works for merling. Here are the grids (squares) to show you how this works. Remember, each pup will get one gene from the mother and one from the father. Also remember that these are ODDS, and each litter does not come out exact (except when 2 solids are mated)!!!!!
1. Here is what happens if you mate two solid dogs (a solid dog has to be mm because solid is recessive)
m mm mm
m mm mm
Mom is on top (mm) and Dad is on the side (mm) and all the puppies are solid, too (mm)
2. Now, here is what happens when you mate 2 merles (all merles, except double merles, have to be Mm)
M MM Mm
m Mm mm
Again, mom on top, dad on the side. Both are merles. You get 50% merles, 25% double merles and 25% solids.
3. Now here is what happens when you mate a merle and a solid.
m Mm mm
m Mm mm
Mom, on top, is a merle, and dad on the side is a solid. There will be 50% merle pups (the same percentage as a merle to merle breeding) and 50% solid pups and NO double merles.
You cannot get a double merle pup from a merle to solid breeding unless the solid parent is, in reality, a "hidden" or "occult" merle.
4. Finally, here is what happens when you mate a double merle to a solid.
m Mm Mm
m Mm Mm
Mom, on top, is a double merle, and dad on the side is a solid. There will be 100% merle pups.
An interesting side note. In January, I went to the Westminster Dog Show. I saw an absolutely gorgeous Blue Merle Aussie boy, with what seemed to me, a bit more white than usual, but not enough white nor any white in the wrong places, to "disqualify" him from being shown and from becoming a "champion". He was clearly a "hearing" dog. In talking to the owner, I found out that the dog was purportedly a double merle, one of the 5% or less who have no visual or hearing impairments (it does happen), and was going to be sent out to stud shortly to mate with a solid and have lots of merle puppies and no solids (which are harder to sell than merles).
By the way, breeding a double merle (MM) to a merle (Mm) would produce 50% double merles and 50% merles, while breeding a double merle to another double merle (MM) would produce ALL double-merle puppies (Yikes)! You can make your own squares to show how these matings come out. It's a good exercise and would make sure you really understand this genetics "lesson."
Kim K - Mother of Hoku (1 ½-year-old deaf red double-merle Aussie), Jade (5-year-old hearing Aussie), Nina (7-year-old hearing Belgie) and Nicki (6-year-old hearing we-don't-know-either), and Foster Mom to Quinn (1 ½-year-old deaf blue double-merle Aussie)
Posted By: kim k Date: Sunday, 10 April 2005, at 11:34 a.m.
Here is how the genetic defect, linked to a double merle inheritance, results in hearing and vision impairment. The same is true for for double dapples (dachshunds), double "leopards" (catahoulas) or whatever you call the color diluting gene which gives dogs of that breed their mottled appearance.
Remember, if a dog inherits only one merle gene, it will be a normal merle (or dapple or leopard). If a dog inherits 2 merle genes, one from each parent, it is what is called a double merle (or a double dapple, etc.).
With two merle genes, there is so much color dilution that the dogs are all, or almost all, white (although some, like my two double merle Aussies, have considerable color, too). Actually, it doesn't matter a bit what color the hair on their body is. BUT the tiny hairs in the inner ear have to have pigment in order to stand up and carry sound waves. And if those tiny hairs lack pigment, as they do in the majority of double merles, the dog will not be able to hear.
The cause of blindness or other eye defects that occur in double merles is not as well defined. However, when the embryo develops, the eyes develop at a site right next to the site which determines the coloration of the dog, so it is felt that this may be the cause or at least part of the cause. My Hoku, my red double-merle Aussie girl, has what are called starburst pupils (that is how she gets her name which is Hawaiian for Star). The iris (the blue or brown portion of the eye) also lacks pigment as a result of the double merle inheritance, and so Hoku's pupils have streaks extending from their center. Her pupils look like the suns that children draw -- a circle with about 5 spikes extending outward. Because it is hard for a dog with starburst pupils to "close down" their pupils in bright light, these dogs often squint in the midday sun. I bought Hoku some Doggles, doggie sunglasses, but she doesn't really like them, and since she doesn't seem to be bothered by squinting.....I tried to upload a picture of Hoku in her doggles. Let's see if it works!
About 90%-plus of all double merle Aussies have either hearing or vision impairments, and some have both. This has led breeders to kill white puppies at birth, leading to the other name of double-merle pups, "lethal whites." The genetic defect itself isn't lethal, and the ONLY genetic defects these dogs have are the hearing or vision impairments -- which do NOT get worse over time by the way. If you want to learn any more about the genetics, etc. there is a great website, www.whiteaussies.com which you can go to. The genetics would be the same for any dog which has mottling in its coloration -- e.g., Great Danes, dachshunds, Shelties, catahoulas, Aussies, etc. By the way, you cannot tell if a puppy hears or not until it is 6 to 8 weeks old.
There is also a "white" gene which is similarly involved in other breeds of dogs, like bulldogs, Dogos Argentinos, Dalmatians, etc. which is different than the merle gene. I do not know as much about that as I do about the merle gene inheritance.
Kim - Mother of Hoku (1 ½-year-old deaf Aussie), Jade (5-year-old hearing Aussie), Nina (7-year-old hearing Belgie) and Nicki (6-year-old hearing we-don't-know-either), and Foster Mom to Quinn (1 ½-year-old deaf Aussie).
Re: How hearing and vision impairment are linked to the double merle inheritance *LINK*
Posted By: Karen, Andy's ^i^ mom Date: Sunday, 10 April 2005, at 12:04 p.m.
Kim, I couldn't get more than the leaves on the photo! probably my computer. My 2 lethals have microopthalmia (undeveloped eyeballs). In one, it is a blind sentence. Plus she has either PPM or dermoid on other (vets couldn't decide-but it looks like PPM). (But, happy to say, she has perfect hearing!) Had it needle aspirated during spay surgery and some "stuff" came out, doc didn't know what it was, but the cyst-like thing is still there for keeps. In the other, GabrielDeafPup, he had some good vision out of microopthalmic eyeballs as a small baby, which is being lost 1) as his head grows, his eyeballs do not, and 2) he is developing PPM on one eye. But can still see a little out of other for a little while longer. He is 9 months now, so should not grow too much more. I am hoping for slight vision at full grown, but I am not optimistic. Do you know of microopthalmia which grown dog has and can still see? BTW, AllicksBlindPup was diagnosed with pthesis bulbi, which I researched like a crazy woman til I found the site below, and did my own diagnosis and found a more knowledgeable vet.
"Field Guide to Aussie Eye Defects" courtesy of AussieLads
In Response To: Re: How hearing and vision impairment are linked to the double merle inheritance (Karen, Andy's ^i^ mom)
"Do you know of microopthalmia which grown dog has and can still see?" FWIW, Domino has a micro eye that can see just fine (it's NOT related to a genetic defect). He's a grown (although small) kitty. He also has no tear duct in that eye, so it runs all over his face.
Posted By: critters Date: Sunday, 10 April 2005, at 12:04 p.m.
"BUT the tiny hairs in the inner ear have to have pigment in order to stand up and carry sound waves. And if those tiny hairs lack pigment, as they do in the majority of double merles, the dog will not be able to hear." OK, makes sense. "The cause of blindness or other eye defects that occur in double merles is not as well defined." Ahh, it sounds like the lethal white phenomenon might be at least somewhat similar to albinism in humans, who are also apt to have vision problems. Perhaps the retina needs pigment, too? " My Hoku, my red double-merle Aussie girl, has what are called starburst pupils (that is how she gets her name which is Hawaiian for Star)." I can't see but the top of tyour picture, but I think I've seen a pic of starburst pupils before. Interesting!!!
Some color examples
Posted By: Karen, Andy's ^i^ mom Date: Sunday, 10 April 2005, at 12:15 p.m.
The link below shows some different examples, and some of the black & reds would also be called tri's. They don't all necessarily meet breed standard (in the amount of white, the encircling of the eye with color and points, etc.) but they are other wise fine, healthy dogs which may be disqualified from the ring. There are also pattern whites which likewise are generally defect free, but just have too much white and are disqualified from the ring. And of course at the bottom, the MMssss This is GREAT! Thank you KIM, CRITTERS AND MARK!!! And Cyndi, and Lorraine and the AZ gang
Again, courtesy of AussieLads, Colors