PHA Informaiton

[virginiajim]

OK, I will start, PHA is a new subject for me and I just wanted to share some information I found to help start some discussion in this area of the site.

www.dextercattle.org/genPHA.htm

What I dont understand is where did it come from if it did not exist or was not detected until the 20th century? Was it a breading mutation that has been introduced to the blood lines?

[cddexter]

Hi Jim.
Dexter Australia is lucky because they have a strong genetics team and a pro consultant available to them for the asking. Dr. Peter Windsor (used to be Harper...long story), U of Sydney, lends himself to their assn for advice. In AU, there were some bloated aborted calves that had normal leg length, so they weren't chondro bulldogs. Peter was consulted. Pedigree backgrounds were checked and they figured out a likely source for the problem--semen imported from Canada--Trillium Chabotte.

Meanwhile in the US, Dr. Jon Beever was looking into aborted Maine/Shorthorn calves which showed edema. He found the dna locus for a mutation which caused three base pairs of amino acids to be missing, which in turn caused the lungs and lymph system to be defective, which in turn caused the calf to bloat with fluid. The mutation was recessive so of course there was no way to visibly identify carriers. It was only when the calf was homozygous for the mutation and it was aborted with all that fluid that you were able to figure out who had it (sire and dam both). The mutation was called pha.

Jon had been working on the Dexter chondro gene about 10 years ago, and had received some dead Dexter calves that didn't check out for chondro (Dr. Julie Cavanagh in AU found the locus first) but thinking ahead, Jon had kept tissue. With this new research he wondered if maybe the problem with the non-chondro dead Dexter calves might possibly be related to the Maine problem. He used some of the saved tissue, and discovered there was a similarity: not the same mutation, but another mutation at the same locus, which caused a similar result. In the case of Dexters, it's 84 base pairs missing--a huge chunk of dna.

AU sent some of their Dexter tissue to Jon, and Jon confirmed it was the same mutation he had identified in US Dexters. Now the hunt was on.

Through pedigree research and the use of dna from a straw of semen, Trillium Chabotte was identified as the source of the mutation in AU Dexters. Chabotte's sire, Aldebaran Priapus was also on AI, so a straw of his semen was checked and he was found to be a carrier, too. That took it back one generation. Unfortunately, Priapus was a very popular AI bull in the early days of Dexters in Canada, and his semen was also exported to the US. All Jon's samples went back to either Chabotte or Priapus, so that confirmed the 'source' here in North America.

Priapus' sire was Trillium Cluny, also on AI. Cluny's semen was checked and was found free of the mutation. Priapus' dam, Woodmagic Wheatear was long dead by now, but one US breeder had had a calf with her behind the pedigree (but NOT Priapus) that appeared to have had edema and normal legs. The cow was checked and confirmed a carrier, taking the mutation back one further generation to the imported Wm cow. Thus the mutation arrived in North America in 1978-9 with the Woodmagic importation to Canada, and spread from there.

In England, there was a lot of 'oh, can't be from here', but of course ...it was. As well, there were English stories of a bull some years earlier who occasionally threw deformed not-chondro calves, and his line came down to Wheatear, so it's likely she got it from him, and it's probably been around for a while.

There used to be stories of aborted calves, which everyone just assumed were chondro bulldogs because that was the problem that was known and expected. Because pha is recessive, and you'd have to have both parents a carrier, and the mutation wasn't all that common, it took a long time for two carriers to be mated and have that 1:4 chance of an aborted calf for it to actually 'show'.

So far the 84-base-pair version of the mutation is limited to Dexters. We don't have any info on when the mutation happened, or which English Dexter started it, beyond knowing it likely that it was present in the 60's and 70's, and common enough by then that English breeders were starting to see deformed calves. I'm thinking that probably takes the mutation back at least another 20-30 years minimum.

Keeping in mind that divine intervention did not create Dexters, man did, and that in the early days as long as the bovine was small, (more or less) solid colored, horned, and had short legs, it was accepted into the foundation registry. The original background could have been anything.

Because only Dexters appear to have this specific form of the mutation, I'm thinking it had to be a genetic glitch within the registered Dexter population and not from 'outside' genetics, and a best guess is it must have happened at least 70-80 years ago. The alternative is that it was present in a foundation animal, and that animal's relatives stayed grade animals and were possibly eaten, or didn't get into another purebred registry, and so have disappeared from the scene. Patti Adams found an old reference dating back to 1905 which could relate, so.....early Dexter breeders could have introduced the gene without knowing it, very early on.

Does this help? regards, c.

[dexterbovinefan]

Yes that helps very much. Are you a historian/geneticist, or just a great story teller?  In any case, bravo!  And thanks for sharing your knowledge.