Ken Rosenfeld
Member # 18
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 Posted 01/17/2009 10:24 AM (Edited 01/17/2009 10:40 AM) |
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It's interesting to look at other animal models for deciphering this particular issue. In horses if one goes beyond the "paints"( includes pintos), which I see as being a kind of random, almost Jackson Pollock type of of pigment distribution programmed in utero and looks in particular at specific melanocyte migration pattterns of distribution, there might be a model for alpacas.
The Tobiano, Sabino and Overo horses each have a distictive pattern of melanocyte distribution and can't be confused with other "multis". The genes have been mapped and each is dominant.Of the three, homozygotes are known from gene mapping and it is only the homozygous Overo that is lethal as a result of:
OLWFS: Overo lethal white foal syndrome (OLWFS) is directly related to the Overo gene (O). One copy of the gene produces an attractive white hair color pattern. Two copies of the gene cause developmental defects in the foals causing it to appear entirely white and to die shortly after birth. As noted above, scientists at the University of Minnesota, University of California and from Australia discovered the cause of the condition based on its similarity to Hirschsprung?s disease. A molecular genetic test is available.
We know that BEWs are not all deaf. This is a result of gene expressivity. The BEW gene results in a white alpaca with lack of pigment in the iris, that sometimes is accompanied by a lack of hair cells (derived from the same elements) in the inner ear, resulting in some , but not all, being deaf.
Many of us are familiar with 2 good models of expressivity:
Marfan's syndrome in humans is caused by the dominant gene that codes for fibrillin, FBN1, an important pathway in the production of connnective tissue. Phenotypically it is very variable, with sometimes minimal change in body type and long slender fingers and "double jointedness", and at the most severe can result in an aneurysm of the thoracic portion of the aorta which is potentially lethal, and literally a powder keg waiting to explode. In turns out that the severity of Marfan's correlates with the position of the FBN1 gene in any one individual. I would presume that , depending on the location, there are some other genes that are successful in a "partial rescue" of the metabolic defect in fetal development.
Several well known groups of cats have a specific polydactyly gene including the "Hemingway cats" in Key West and the Danforth cats in the British Ilses.Danforth studied this dominant gene for 55 generations and found that the number of extra toes was very variable in any one cat from generation to generation, with 8 toes not being unusual.
So if there is a lethal outcome in utero from a dominant, homozygous lethal grey, it is not likely to be genotyoically based as much as by phentypic expression. Like the OLWFS type in the Overa horse it is likely a result of neither, blue eyes or deafness, but a critical developmental path in utero of an organ whose development is essential to in utero progression. Much the same as not all BEWs are deaf, perhaps not all homozygous roans are lethal.
If in fact the roan gene is dominant, one would also have to speculate that "modern greys" are never at risk for BEW, deafness and in utero death. What we can take from deafness in BEW is that melanocytes, or the hormone (substance) that controls their migration like have a role in fetal development associated with particular functions or events in organ development.
This is already well known in humans. We know that there are many hormones, either from mom or placenta that are essential to fetal growth and organ differentiation, beyond the commonly known adult function (sommatotropins, melanotrophic hormones etc).
Best know is insulin Diabetics, particularly those with poor control and hyperglycemia (high blood sugars) in the first trimester are at increased risk for birth defects, (especially cardiac) and fetal demise and consequent miscsarriage.
So I think the answer might well be "perhaps", "sometimes", depending not on the genotype of the the homozygous roan but the expression of the melanoctye migration pattern.
There may be a good explanation for the apparent discrepencies in the predicted rates of fetal loss and and color outcomes so hotly disputed as testimony to in lethal grey genetics.
Increasingly, I'm thinking out my breeding decisions in grey, with both penetrance and expressivity in mind.
Best to all,
Ken
Kenneth M Rosenfeld MD
Mt Aukum, Ca
www.renaissanceridgealpacas.com
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mpcpneilp
Member # 6
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| Posted 02/09/2009 09:40 PM |
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Ken,
You can add one more animal model to the homozyous lethal column. Roan as a color in hampsters is a homozygous lethal.
But all of this is beside the point. Several folks (Liz Paul and Andy Merriwether included, see their articles in the "Articles" section) have theorized that gray is a homozygous lethal in alpacas. This is a nice theory, the question is does the theory hold up in the real world?
I would answer it likely does not.
Both Liz and Andy cite Register and ARI data showing 2/3rds of the offspring of "gray" alpacas are gray, which supposedly supports the homozygous lethal model. This would only be true if all "grays" in those registries were tuxedo grays. This is certainly not the case in ARI and I doubt it is any more true in the Aussie Registry.
Registry database data contradicts the homozygous lethal model. Further, if gray were a homozyous lethal there would be 25% more fetal deaths in tuxedo gray to tuxedo gray breedings. This is not observed even by Liz Paul. I've measured such outcomes and have found tuxedo gray to tuxedo gray breedings to be successful just as often as pairings of other colors.
Last, we all accept that there is great variability in the expression and perhaps the penetrance of the tuxedo gray pattern. Given that the pattern itself is so variable, how would we even recognize a homozyous gray in the first place? The arguement that there are no animals that always produces gray no matter what they are bred to is questionable when we already acknowledge that tuxedo gray doesn't always present itself as.......tuxedo gray.
I would agree that a final verdict is still out on this issue, but available experience, especially the lack of an increased fetal death rate in tuxedo gray to tuxedo gray pairings, makes the homozygous lethal theory in tuxedo grays unlikely to be true.
I'm looking forward to your dominant roan article roan Ken. Stir the pot good and hard 
Neil
A Paca Fun Farm
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