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  Discussion on CSNB in Appaloosas with Homozyous Coat Patterns
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Member:
mitma

Posted on Sunday, Nov 4, 2007 - 11:30 am:

HA Friends and Dr. O,
I belong to another great educational internet horse discussion group, the Appaloosa Project, which focuses on the genetics of the leopard complex (LP), i.e., the main gene that gives Appaloosas or other spotted breeds their coat patterns (www.appaloosaproject.info). This group recently announced the publication of results from one of their research projects in the Journal of the American College of Veterinary Opthalmologists (Veterinary Opthalmology, Volume 10, Issue 6, pages 368-375, November, 2007) which examined the prevalence of CSNB in Appaloosas... Though the study examined only 30 horses, the findings were quite remarkable, in that, 100% of horses demonstrating a homozygous coat pattern (i.e., horses with coat patterns associated with having inherited TWO copies of the LP gene from their parents) had CSNB, whereas, other horses who had only one copy of the LP gene (heterozygous LP/lp horses) or NO copies (i.e., lp/lp horses, non-Appaloosas or solid, non-characteristic horses born to Appaloosas) had NO EVIDENCE of CSNB! Examples of homozygous LP coat patterns (LP/LP) include few-spot, near few-spot, or snowcap horses, whereas examples of heterozygous coat patterns (LP/lp) include varnish roan, snowflake, blanket with spots, leopard, or near leopard horses. Those familiar with the genetics of Appaloosa coat patterns also probably know that the AMOUNT of white visible in the coat relates not only to the presence or absence of the dominant LP gene, but also, on the presence or absence of other "modifying" genes, such as the presumed major modifying gene known as PATN1. Though there is not yet a known genetic test to determine the presence of LP or PATN1, this study may still have implications for strategies utilized by breeders of Appaloosas or other spotted breeds of horses... I hope that in bringing this information to HA that I have not made any mistakes in my understanding of the issues... I will refer everyone to the original article or the Appaloosa Project (www.appaloosaproject.info) which similar to HA.com allows the public to read various bits of information, but requires a subscription in order to participate in online discussions, etc...
Martha
P.S. Dr. O, please let us know what you think of this article...
Moderator:
DrO

Posted on Sunday, Nov 4, 2007 - 8:11 pm:

Hello Martha,
While I was composing a review of this very research summary you were posting this review. I did not have the whole article to review just summaries so look forward to reading the whole article. You will find my initial review it on this same discussion page.

One thing that confuses me about your post is that if they cannot test for the Lp gene allele how do they know the allele pattern of the participants? Maybe this information is in the whole article.
DrO
Member:
mitma

Posted on Sunday, Nov 4, 2007 - 10:14 pm:

Hey Dr. O,
I haven't read the whole article either, just the abstract, and of course, the other website's announcement to it's members... But, despite the fact that there is not a genetic test for the presence of LP, "existence" of it is usually accepted/inferred from a horse's phenotype, i.e., coat pattern. Apparently, in the study the authors used Appaloosa horses that demonstrated phenotypes which unambiguously correspond to either the LP/LP or LP/lp genotypes. And, specifically, for these two "appy" groups, they chose horses with high levels of white patterning, so as not to have any "dispute" over assigning the genotypes. Their third group studied consisted of solid, non-characteristic horses (lp/lp).
LP/LP horses can also be identified from progeny records, though this method is usually more applicable to stallions (i.e., to be able to assess large numbers of offspring),
(Note, I am using LP to designate the form of the allele positively associated with the Appaloosa coat pattern, though in the paper, the authors may have used Lp; and, of course, lp designates the absent form of the allele, i.e., NO Appaloosa patterning or characteristics)...
I guess what I'm saying is that the patterns of inheritance in offspring from Appaloosa to Appaloosa breeding vs. Appaloosa to non-Appaloosa breeding are so well studied, one can ascertain the LP status of these horses... One of the scientists who actively directs the educational site I refer to above, Sheila Archer, does a wonderful job explaining the statistics of coat pattern inheritance (using Punnett squares, etc... ?sp)
For instance, if I breed my Appaloosa mare (she's a varnish roan pattern, therefore, heterozygous) to a snowcap pattern stallion (therefore, homozygous), then the resulting outcomes would be: 50% chance of a homozygous "appy" foal (LP/LP) and 50% chance of a heterozygous "appy" foal (LP/lp)... "theroretically", either of those outcomes would be a foal that manifests an obvious Appaloosa coat pattern, though the presence or absence of the other known/suspected modifying genes would certainly affect the foal's phenotype. And, on the other hand, if I breed my mare to a heterozygous stallion (say a leopard coat pattern stallion, like the one she was bred to in 2006), then the resulting possible outcomes are: 25% chance of a homozygous "appy" foal (LP/LP), 50% chance of a heterozygous "appy" foal (LP/lp), and 25% chance of a solid, non-characteristic, "non-appy" foal (lp/lp). And, that's exactly what she delivered this past spring was a beautiful, solid bay colt that (to date) has NO obvious appy characteristics.
I think that most "good" Appaloosa breeders (like any reputable, ethical, and responsible horse breeders of any breed) are concerned most with breeding for performance and personality, but the "color" aspect adds a little extra fun to the process... and, of course, now the implication of a significant medical condition...
Martha
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