Posted by carol davidson on December 18, 2002 at 21:07:05:
This is in reply to the various messages relating to the three topics, back in September/October:
Dexters carry all three coat colours at the base colour locus, and a fourth colour found at a different locus. The three are Black (dominant), red (recessive) and a wild gene whose colour varies between breeds but is consistent within a breed. In Dexters, wild is RED. Black is dominant over both red and wild. We also have another colour mutation found at a different locus which is recessive and produces DUN (well, actually it's BROWN, but I don't think we are going to change it now--especially when there is Brown Swiss, which isn't brown at all.
For those who think Beryl did something funny all those years ago, the same dunbrown genetics is found throughout U.S. herds, where there is NO upgrading, and animals relate back to original Irish and English imports from La Mancha, Gort, Castlelough, etc. around 1910 and have NEVER seen a Woodmagic animal. Recent genetic research has unearthed the locus for this gene
The other breeds who appear brown/dun have ben checked and NONE of them carry this mutation. This means Dexter dun is a unique-to-the-breed mutation, and it has been around for a long time (certainly as long as there have been animals identified as 'Dexters').
Andrew Sheppy briefly cast a glance toward a 'brown' gene but discarded it in favour of mis-registered crossing, going on at length with his 'facts' about Channel Islands influence. Hopefully this DNA proof will silence the dun detractors and straighten out the theorists. It's actually quite amusing to think that for all the criticism and misrepresented facts, if one wants a truly rare animal, DUN Dexters are at the top of the list.
The base coat colour locus for duns can be black/black or black/red or black/wild. It is predicted that red and wild will dominate the dun gene, if both are present in the homozygous state.
It is genetically impossible for two reds to produce a black. More likely, some other bull (uncut calf?) was the real sire, and the breeder just didn't know it, or it could have been dun which has a black gene. I found several cases of this when I was researching the colour issue originally and worked with Wantsley Rob Roy and Barnston Mickey Mouse.
For more information generally, contact me. For detailed genetic information, contact Sheila Schmutz at the University of Saskatchewan, in Canada. There is a test available for dun, but it has not been picked up by any of the labs (probably because the low interest would make it eneconomical to run).
The dwarf gene is actually a lethal defect found on one of the bone growth loci. In Dexters, a mutation which caused an extra amino acid to be inserted near the front of a 'sentence' that instructs the body to make bone caused a 'full stop' terminator to be generated, causing the rest of the sentence to be ignored. This means that the hetero (one gene of each type) carrier (or a shortleg as it is euphemistically known) has 3/3 bone growth istructions on one of the pair of genes (from the normal or longleg parent) but only ~1/3 bone growth from the other gene of the pair (from the carrier or shortleg parent). I.e. dwarfs (shortlegs) only have 4/6ths of their normal bone growth. In her research, Julie Cavanaugh found there was an average difference of roughly 5 inches between the non carrier and carrier females, and 8 inches between the two in males. In the case of bulldogs, the severity of the genetic disease is so great, the resulting embryo is non-viable, and is aborted, either very early in the term, around the 30-50 day mark, or later at around 5-8 months.
For those of you who do arithmetic, this means that for your non-dwarf cows to meet the height standards of the DCS, there should be a maximum dwarf height standard of 5 inches shorter. Now there's an eyeopener but it certainly points out that 'tall' non-dwarfs aren't the problem with the breed, it's the breeder who uses big dwarfs and thus perpetuates the heights in the half of the population that is genetically normal.
I know there has been a tendency to refer to short, medium and long legs, but there are only TWO types: either they carry the dwarf gene or they don't. The 'medium'legs are either smaller-end non-dwarfs or they are the more proportionate looking dwarf, without the big head and pot belly.
For those who want to breed just dwarfs, it isn't possible because the gene you are using to create the look you like is what is known as a 'lethal'. Real Dexters, as originally selected, will never breed true.
You either breed two dwarfs and live with the bulldogs and cull the normal calves, or you breed non-dwarf to dwarf, and cull half your calf crop. No matter what you do, you can't exceed a 50% dwarf outcome.
Re polling: it is a dominant gene in cattle. One of you thought it recessive, but this is not the case. Perhaps you were confusing it with the polled gene in sheep, where it is recessive.
If you find polled genetics confusing, use dominant Black and recessive red. The genes worth the same way and the percentages are exactly the same.
If you are concerned with upgraded genes, scientists are of the opinion that Godstone Esmeralda, the source for one of the polled lines in England, has a mutation involved. If you want more info, contact me.
Regards, carol davidson.