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Sunday, January 5, 2014

The Science of Horned-Polled Status 101 - Facts & Reminders


 Article by Catherine Brown , as seen in the Canadian Hereford Digest - Commercial Edition - October 2013

The Following questions are answered by Dr. Shela Schmutz, with additional commentary and deductions by the author, with Schmutz’s guidance.  Dr. Schmutz is considered a leader in the field of animal genetics.  She is a professor in the Department of Animal and Poultry Science at the Unicersity of Saskatchewan and is a researcher of international acclaim.  Areas of interest and specialty include genetics of hornes and scurs in cattle.

If a cattle are scurred, does that mean they are horned?
Can a horned animal also be scurred?
“Well….yes….A horned animal can have the gene that says it should have scurs but since horns and scurs grow in exactly the same location, if the animal already has horns, then it can’t grow scurs too.”
Can a polled animal be scurred?
“Yes.  That’s why some cattle people prefer to say “smooth polled” [in the absence of scurs,] to be clear that the animal is polled, without scurs.”
What is a Heterozygous polled or horned animal?
“The term “heterozygous” means that the animal carries only one copy of one allele and one copy of the opposite allele. A heterozygous polled or horned animal would therefore be labeled (P/p).”
The animal may or may not have scurs. But only heterozygous polled cattle ever grow scurs. Two scurred alleles are required in females and only one in males, making this a sex-influenced trait.
A heterozygous polled animal is polled but still carries the horned allele and can therefore pass horns along to progeny. On the other hand, a heterozygous horned animal is also polled but can influence its progeny to be either polled or horned.
What is a Homozygous polled or horned bull?
The term “homozygous” means that the animal carries two copies of one allele (either the polled or horned), ensuring that genetically, that animal is absolutely polled or absolutely horned. If an animal is known to be either homozygous polled (P/P) or homozygous horned (p/p), it will not have scurs but each can still have one or two alleles for scurs.
A homozygous polled animal can only influence its progeny to be polled while a homozygous horned animal can only influence its progeny to be horned (keeping in mind that progeny’s horned status is also influenced by the dam or sire on the other side of the equation).
Some ranchers use the term “double polled” to mean that the animal is descendent from two polled parents. But this does not ensure the progeny will be homozygous polled, or even polled at all.
However, mating two homozygous horned animals will ensure homozygous horned progeny (p/p x p/p can only result in p/p offspring).
According to an early study of fetal anatomy in Germany, horns seem to begin growth at four months gestation. But 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.
Polled is an autosomal dominant trait in cattle. Autosomal genes are genes present on any chromosome other than the sex chromosome. And because it is dominant, it only requires that one parent has passed it on, in order for it to be expressed in progeny.
The gene causing the absence of horns is at the top of cattle chromosome 1.
Scientists use the term “epistatic”. Epistasis is the suppression of one gene by the effect of another gene. The homozygous polled genotype (P/P) is completely epistatic to the gene for scurs in both sexes.
The gene for scurs is located on a completely different chromosome (chromosome 19), than the gene for polled.
Scurs typically do not appear until about four months of age and stop growing at a few inches, if left on.
“If dehorning occurs too early, scurs can be missed,” says Schmutz.
Some scurs, often called “scab scurs” are never much bigger than a thumbnail. Horn growth makes 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.
All female cattle that have scurs are considered to be homozygous scurred (Sc/Sc). But males can either be homozygous scurred (Sc/Sc) or heterozygous scurred (Sc/sc), if they have scurs.
As Larry Thomas reported in the December 2000 Canadian Cattlemen magazine, University of Alberta professor Mick Price and others looked back on historic records of its Kinsella synthetic beef herd, comparing polled and horned cattle on growth, carcass and reproductive traits. “These and other studies found no major differences,” he says.

The name for the absence of horns in cattle.

Small horn-like growths on the frontal bone in the same locations where horns would grow. Scurs are referred to as “wiggle horns” in German and indeed, most are movable and are not attached firmly to the skull.

One of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.
Examples for the polled gene and scurred gene:
P  = polled allele
p  = horned allele
Sc = scurred allele
sc = allele for no scurs