Sunday, February 24, 2008

Polled, Scurred and Horned

There are 3 different sets of genes that determine whether an animal will be polled, scurred or horned. Two of them can cause horns, and the other causes scurs. One of the horn genes, and the scur gene are both sex-linked, meaning that the genes are linked with the genes that determine the sex of an animal, and they express themselves differently in females than they do in males.

The first gene I will discuss is commonly found in Bos taurus cattle, that is the European and British breeds. This is the Polled gene, P and p. P is polled and p is horned. This gene is NOT sex-linked, and expresses itself in the same way in both male and female animals.

P is dominant, which means that as long as it is present the animal will be polled. PP and Pp animals are both polled in appearance, however Pp animals may pass on either P or p to their offspring. pp animals, are horned in appearance, and will always pass on the p gene to their offspring.

PP = Homozygous - Animal is Polled

Pp = Heterozygous - Animal is Polled

pp = Homozygous - Animal is Horned

A homozygous horned animal, in this animal the horns are present. Note the shape of the poll.

Another Homozygous horned animal, this animal has been dehorned, but again notice the shape of the poll.

This animal is heterozygous polled. She carries the horn gene, but since polled is dominant the animal is polled in appearance. Note the shape of the poll.

And finally, a Homozygous polled animal. Again note the shape of the poll.

As a side note, this gene is one that is known to mutate fairly frequently, which may result in the occaisonal polled animal being born to a horned animal.

The other horn (African horn gene) gene is known in Bos indicus animals, or breeds of Zebu type animals. Brahman animals may carry this gene. This gene IS sex-linked, and expresses iself differently in females than it does in males. It is written as Af and af. In females, Af is recessive, and requires that there be two copies of Af for the animal to be horned. However in males the gene is dominant, and only one copy of the gene is necessary for the trait to express.

In males
AfAf = Homozygous - The animal is Horned

Afaf = Heterozygous - The animal is Horned

afaf = Homozygous - The animal is Polled

In females
AfAf = Homozygous - The animal is Horned

Afaf = Heterozygous - The animal is Polled

afaf = Homozygous - The animal is Polled

A horned female will always give birth to a horned male, however, her female calves may or may not be horned, depending on what gene was passed on from the sire.

The final option for horn like growths is the scurred gene. Scurs are a hornlike growth that are loosely attached to the head. This gene, is thought to work the same way as the Af gene. With the belief that for a male that is heterozygous for the Sc gene to express scurs, he needs to also carry a copy of the p gene. Sc is recessive to the p gene, where a horned animal may be scurred, but they will not be expressed.

So in males
ScSc = Homozygous - Animal is Scurred

Scsc = Heterozygous - Animal is Scurred, although it is believed that HE must also carry a copy of p

scsc = Homozygous - Animal is Polled

In females
ScSc = Homozygous - Animal is SCURRED

Scsc = Heterozygous - Animal is Polled

scsc = Homozygous - Animal is Polled

Again, a Homozygous female should always have a scurred male calf, and her female calves may or may not be scurred, depending on the sire.

In the case of these three genes an animal may possibly carry or even be homozygous for the horn gene, the African horn gene and the scur gene.