Post by Cascade Meadows Farm - Kirk on Nov 3, 2012 12:23:26 GMT -5
Where do polled genes come from? How did the polled gene arise in cattle breeds including Dexters, Herefords, Simmental, Shorthorns, Holsteins?
"The p+ (horned) gene appears to mutate to P (polled) at a fairly high frequency, as indicated by the periodic production of polled animals from horned parents. One recent example was the polled Simmental bull, Polaris, whose parents were both horned fullblood Simmental. The polled mutant has also occurred in Herefords, resulting in the Polled Hereford breed, and in the Shorthorn, Holstein, and Brahman breeds."
"Polled Herefords were developed from the horned Hereford breed which was founded in the mid-18th century by the farmers of Hereford County, England. Among the horned Herefords an occasional calf would be born which did not develop horns. This change from parents' characteristics is known as a "mutation." These cattle soon came to be called "polled," which means naturally hornless."
While the horned genes are complex and little understood, horn growth is almost certainly controlled by a complex set of polygenetic genes including genes for horn size, genes for horn shape, genes for horn color, genes, for timing of growth, genes for angle of growth, etc..
It is likely that the "polled" gene, is not really a "polled" gene, but rather is simply a broken (accidentally altered) version of one of the many genes that control horn growth. If this particular horned growth regulator gene gets "broken" (via chance mutation) then no horns are produced, even though all the other genes controlling horn growth are still present.
This is similar to the "Dun gene" in dexters. There really is no "Dun gene", but instead, dun is the result of a "broken" gene (tyrp1) that is required to finalize the production of black pigment. If both of the tyrp1 genes are broken, then an animal's black pigment remains incomplete and looks brown.
In nature, broken genes can offer advantages in some environments so "broken" genes can be beneficial in certain circumstances. So don't think of "Broken" as bad. In fact, solid black (ED), is a "Broken" (mutated) version of the original E+ wild-type coloration genes from all ancestral cattle.
Mutations typically occur when eggs or sperm are formed and genes are copied. Mutations are simply dna copying errors. Considering how complex the dna copying process is, it's a wonder that there aren't more dna copying errors. A bull creates millions of sperm each day, and it's easy to see how an occasional sperm could have a dna-copy-error in it.
When you understand the DNA copying process and the occasional copy errors, it's easy to see how an occasional hornless calf, can be born to two horned parents. Many folks estimate the frequency to be about 1 in 10,000 calves (perhaps even more frequently, but often goes un-noted by unaware amateurs). When such a polled calf is born to horned parents, he /she is able to pass the "polled gene" (damaged/non-working horn growth regulator gene), to its offspring.
In wild-cattle, such a broken gene would be considered a "Genetic Disease" because a wild cow without horns would be at a disadvantage. But in modern herds of domestic cattle, the broken horn regulator gene is often considered as a favorite feature by those who prefer hornless cattle.
www.ansi.okstate.edu/breeds/cattle/polledhereford/index.htm
www.braunviehcenter.com/cattle_genetics.html
PS. I personally LOVE the look of a bull with horns and I'm happy that some folks still breed horned dexters, but my personal choice is polled, for safety reasons (and I love how they look too).
"The p+ (horned) gene appears to mutate to P (polled) at a fairly high frequency, as indicated by the periodic production of polled animals from horned parents. One recent example was the polled Simmental bull, Polaris, whose parents were both horned fullblood Simmental. The polled mutant has also occurred in Herefords, resulting in the Polled Hereford breed, and in the Shorthorn, Holstein, and Brahman breeds."
"Polled Herefords were developed from the horned Hereford breed which was founded in the mid-18th century by the farmers of Hereford County, England. Among the horned Herefords an occasional calf would be born which did not develop horns. This change from parents' characteristics is known as a "mutation." These cattle soon came to be called "polled," which means naturally hornless."
While the horned genes are complex and little understood, horn growth is almost certainly controlled by a complex set of polygenetic genes including genes for horn size, genes for horn shape, genes for horn color, genes, for timing of growth, genes for angle of growth, etc..
It is likely that the "polled" gene, is not really a "polled" gene, but rather is simply a broken (accidentally altered) version of one of the many genes that control horn growth. If this particular horned growth regulator gene gets "broken" (via chance mutation) then no horns are produced, even though all the other genes controlling horn growth are still present.
This is similar to the "Dun gene" in dexters. There really is no "Dun gene", but instead, dun is the result of a "broken" gene (tyrp1) that is required to finalize the production of black pigment. If both of the tyrp1 genes are broken, then an animal's black pigment remains incomplete and looks brown.
In nature, broken genes can offer advantages in some environments so "broken" genes can be beneficial in certain circumstances. So don't think of "Broken" as bad. In fact, solid black (ED), is a "Broken" (mutated) version of the original E+ wild-type coloration genes from all ancestral cattle.
Mutations typically occur when eggs or sperm are formed and genes are copied. Mutations are simply dna copying errors. Considering how complex the dna copying process is, it's a wonder that there aren't more dna copying errors. A bull creates millions of sperm each day, and it's easy to see how an occasional sperm could have a dna-copy-error in it.
When you understand the DNA copying process and the occasional copy errors, it's easy to see how an occasional hornless calf, can be born to two horned parents. Many folks estimate the frequency to be about 1 in 10,000 calves (perhaps even more frequently, but often goes un-noted by unaware amateurs). When such a polled calf is born to horned parents, he /she is able to pass the "polled gene" (damaged/non-working horn growth regulator gene), to its offspring.
In wild-cattle, such a broken gene would be considered a "Genetic Disease" because a wild cow without horns would be at a disadvantage. But in modern herds of domestic cattle, the broken horn regulator gene is often considered as a favorite feature by those who prefer hornless cattle.
www.ansi.okstate.edu/breeds/cattle/polledhereford/index.htm
www.braunviehcenter.com/cattle_genetics.html
PS. I personally LOVE the look of a bull with horns and I'm happy that some folks still breed horned dexters, but my personal choice is polled, for safety reasons (and I love how they look too).