The Genetics of Polled/Horned/Scurred in Cattle
a brief review about the genes controlling cattle horns or scurs
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|Polled is the name for the absence of horns in cattle, sheep and goats. "Smooth polled" means that the animal has never had any horns or scurs. Some ranchers use the term "double polled" to mean that the animal was descended from two polled parents, but this does not ensure that the animal is homozygous polled, only that it could be homozygous polled. Since it appears this gene has no deleterious side effects and many ranchers prefer hornless animals, it is a desirable trait in cattle.|
|Horns apparently begin growth at 4 months gestation. Nevertheless, in many breeds horn buds are very tiny at birth. If left unremoved, horns continue to grow in size throughout the life of the animal. Shape of horns varies widely among breeds - consider the Texas Longhorn, pictured at the left, versus the Horned Hereford or other shorthorn breeds such as the crossbreds pictured below.|
Polled is an autosomal dominant trait in cattle. The gene causing the absence of horns is at the top of cattle chromosome 1. Although many DNA markers have been mapped near this trait, the exact gene is yet unknown. Some older literature suggests there was a different gene causing horn development in Brahma or Zebu type cattle (Bos indicus)). However, a study at Texas A & M mapped the gene for horns in Brahma cattle to the top of cattle chromosome 1 also. Since this gene has not been found either, it is impossible yet to say if there is only one gene or two genes near each other controlling horn development in Bos indicus and Bos taurus.
Unfortunately polled does not always mean that the head will have no scurs since they are caused by a different gene. In some breeds, this term can only be used if no scurs occur but in others it just means there were no horns.
Polled Research From Other Groups
Although several labs reported that the gene for polled mapped to the top of chromosome 1 before 2000, no group was able to discover the gene or mutation causing this phenotype in cattle. Then about 2012, researchers in Germany and France began actively conducting research again and progress has been made.
In addition, these research groups have now shown that the polled mutation has occurred more than once. This helps explain why not all labs reported mapping the trait to exactly the same position and also why it has been so difficult to find the mutation causing polled.
The German group reported a 202 bp duplication that is present in many beef breeds. This allele has been referred to a Pc or the celtic polled allele by the French researchers. It is not in a known gene, but is in the general vicinity of the OLIG1 gene on cattle chromosome 1.
There appears to be at least one different mutation in Holstein cattle that are polled, if not more. A specific mutation has not yet been identified in most Holstein cattle, but a set of markers, or a haplotype is present in many. The French are referring to this as the PF or polled Friesian allele. One group suggests that a family of German Holsteins had an entirely different mutation in the interferon gamme receptor 2.
The bull at the left has very small scurs. He is a one year old crossbred Simmental/Belgian Blue.
Scurs are small horn-like growths on the frontal bone in the same locations are horns would grow. Scurs are referred to a "wiggle horns" in German and indeed, most are moveable and are not attached firmly to the skull. Scurs typically do not appear until about 4 months of age and stop growing at a few inches if left on. Some scurs, often called "scab scurs" as never much bigger than a thumbnail. Horn growth would make it impossible for scurs to develop at the same spot but horned animals can carry the gene for scurs.
Traditionally the scurred trait has been reported as sex-influenced. Male cattle need only one allele for scurs to exhibit the trait, whereas females need two alleles. Long and Gregory detailed the inheritance very well back in 1978. They suggested that homozygous polled masks scurs, unless the animal is alsohomozygous for scurs.
Asai (2001) found evidence at the DNA level to prove scurs are sex-influenced. Her data also suggest homozygous polled masks even homozygous scurs - i.e. is epistatic. See the table below.
Asai's studies proved that the gene for scurs is not on cattle chromosome 1, but on cattle chromosome 19. Therefore, as long believed, this is an entirely separate gene than the gene causing polled.
A French group of researchers suggested that scurs were inherited differently than previous researchers in 2009. However, in 2011, the same group reported that the scurs in the Charolais cattle they had been studying had an atypical form of scurs, they refer to as "bovine type 2 scurs syndrome". They found that a mutation in the TWIST1 gene is responsible for this type of scurs.
Articles in the Popular Press and Links to Other Sites on this Topic
for further information contact:
Department of Animal and Poultry Science
University of Saskatchewan
Saskatoon, Canada S7N 5A8
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