Ken Rosenfeld
Member # 18
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 Posted 01/16/2009 04:04 PM (Edited 01/16/2009 04:25 PM) |
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Rosenfeld's Hypothesis
Roaning is a dominant gene, with extremely variable penetrance in all it's manifestations, but dependent on the base color, and pattern for it's expression.
In this respect, it doesn't appear to be too much different than color genetics in horses.
The highest degree of penetrance is observed in the tuxedo or multi pattern and lowest over whites,fawns or browns which is why many show up as "indefinites" or poorly expressed as has been alluded to in fawns. I do recognize that this is more likely an example of poor "expressivity" rather than penetrance.I think that we will show that even "modern" grey bred to appaloosa has a higher than predicted rate of "grey appaloosa" and that they are not as 2 separate recessive genes, but a dominant grey covering a recessive appaloosa, or even harlequin if you like.
A quick review of my herdsire's (Guns n' Roses) first 14 breedings (whom I believe both Neil and Andy agree is a tuxedo rose grey, but "poorly" expressed tuxedo, reveals that of 2 breedings to classic grey he has produced "classic" grey. In one breeding to fawn, he produced a "classic" rose grey (Bonanza, on my site). Two fawn dams--->2 "modern medium rose greys" (Cat Ballou, Delilah), 1 brown dam---> "modern, light rose grey"(Gold Panner), 1 true black dam---> "modern, dark silver grey" (Desperado).
By the way, I have to side with Andy and agree that any incomplete melanocyte migration, even if a single boot and white splash on face is a "tuxedo". I just don't see where one can draw a very arbitrary on the "mostly white face" criterion for "classic" grey as suggested the the AOBA changes this year,and wish the judges checking color well this season.
So far, with my grey herdsire, the transmission is 50% (i.e.7/14) grey, but only 3 tuxedos 2 out of tuxedo dams, 1 out of solid. The 4 "modern" greys are all out of solid females(2 fawns, 1 brown, 1 black) and if roan were recessive, the likelihood of me having 4 roan carrier females out of 11 solid females, given the presumed incidence of a roan carrier state, would be extremely remote.
As you know, even in conditions such as osteogenesis imperfecta, the mechanisms of gene penetrance are poorly understood.
I suspect that a dominant "roaning" gene is more common than the 9% phenotypic greys that we see. It likely exceeds even those that would be lumped in with indefinites.The most confusing example that I know of is Snowmass Peruvian Casanova, who is out of 2 white (unfortunately) imports, one being Bueno. I say unfortunately because I would love to know who Bueno and Angelique's ancestors were.
It suggests to me that penetrance is least in solid white, then fawn, then brown and black and ultimately the most common being pattern and dark spot genes, to include a tuxedo gene.
So in a sense it might beg the question:
Are tuxedo greys, really poorly expressed "modern" roans?I say that tongue in cheek, because it summons up to me to me the image of having to breed patterns to get white.
It would also suggest to me, that other things being equal, that if one is interested in breeding for greys (roans), it might be important to find a sire with high penetrance for the roan gene rather than focus on tuxedo or other patterns.
I also found it interesting of the different roan patterns in roan horses (Tobiano, Sabino, Overo), all of which have been gene mapped and established as dominant, that it is only the Overo, that has found to be lethal when homozygous, with multiple known congenital defects. Maybe there is a subset of lethal grey within tux grey breedings that is lethal homozygous, if there are any at all.
I am working on an article that I hope will continue to perhaps add some data to this hypothesis. I think that I can make more persuasive with the use of pictures, rather than having to sift through my website. I will have at least 25 more offspring to contribute this year, and many of these will be breedings to "classic" grey dams. Most importantly, I'm looking forward to seeing data form breeding results out of my current crop of "modern" grey male cria.
Go grey!
Ken
Kenneth M Rosenfeld MD
www.renaissanceridgealpacas.com
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mpcpneilp
Member # 6
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| Posted 02/01/2009 07:50 PM |
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Ken wrote;
"I just don't see where one can draw a very arbitrary on the "mostly white face" criterion for "classic" grey as suggested the the AOBA changes this year,and wish the judges checking color well this season."
Ken,
The way it happened was there was a recommendation made to the SRC that we split tuxedo from non-tuxedo. The 10 SRC members in the room spent about half a day figuring how this could be done (where a line could be drawn). Eventually the current definition was hammered out and then checked against pictures (many many pictures). It turns out that the single most reliable factor is a mostly white face.
I also have no problem with "gray fleece and a white spot" as a highly sensitive test. It is however not very specific. Roans with a spotting gene would all be included in this definition.
Best regards, and I look forward to your complete article,
Neil
A Paca Fun Farm
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