(Condensed information about genetics pertaining to horn growth in cattle; written for personal use.)
Horn growth is sectioned into three catagories: polled (without horns, naturally), horned, or scurred (marked by detached, horny tissue). These terms can also be used to describe sheep and goats, however, this writing focuses on how these genes affect cattle.
Hornless cattle are generally more desirable in the beef and dairy industry, especially in large operations, simply because it deprives an already intimating animal of formidable weapons. Disbudding, dehorning, and breeding for the polled gene are how one goes about achieving this. Disbudding refers to horn removal done on calves and involves cauterizing the horn buds. This destroys the tissue, preventing growth, and is usually preformed while the calves are young, around the same time as castration. Dehorning is the removal of a fully developed horn and should be done after the animal has been anesthetized.
Scurs are genetic, but will also occur if all horn tissue is not properly removed, resulting in horn tissue growth that is detached from the skull but held in place by skin.
From an ethical standpoint, it should be noted that, according to research compiled by the American Veterinary Medical Association (AVMA) on disbudding and dehorning pertaining to animal welfare (viewed here), it is understood that both these methods are painful to the animal they are preformed on, however, it is still widely recognized in the commercial industry as a necessary procedure for human and animal safety.
When breeding animals, especially for profit, it is encouraged to have an understanding of genetics, both to improve and ensure a strong bloodline, and to offer a quality product to the public market. Generally speaking, determining the polled or horned gene is a much easier process compared to breeding for other traits, as there are fewer genes involved.
Cattle carry two genes for the polled/horned trait. There are three possible genetic outcomes: homozygous polled (PP), or two polled genes, homozygous (pp), two horned, and heterozygous (Pp), meaning a carrier of both polled and horned. The polled trait is dominate over the horned trait, and because of this, homozygous polled(PP) and the heterozygous(Pp) cattle will be polled. Homozygous(pp) will be horned.
Scurs are another possible outcome. As mentioned previously, scurs are horn tissue that are connected to the skin rather than skull. They can appear as scab-like marks on the cattle’s head, or incomplete horns. Scurs can be “wiggled”, much like a loose tooth, whereas a complete horn will not move at all. However, at young ages, the horn tissue is still growing, so it is a possibility that developing horns could be mistaken for scurs.
The scur gene is sex-linked, being dominate in bulls and recessive in cows, and are only expressed in polled cattle, as the condition is covered by horns in homozygous (horned) breeds. Therefore, polled cattle can be referred to as “smooth polled” or “scurred polled”. Cattle that are heterozygous(Pp) will express the scurred trait, so homozygous(pp) and homozygous polled(PP) will most certainly be free from scurs. There are 13 breeds of naturally polled cattle. Aberdeen, also know as Black, Angus is the most popular.
As of this writing, September 4th, 2015, the polled gene has not been identified, so DNA testing is not yet a viable option for those invested in breeding hornless animals. Matings are the current sure-fire way to test for polled.
There are no negative side effects in the inheritance of any of these aforementioned genes specifically. Polled cattle are solely a convenience for owners, mainly because raising polled cattle eliminates the need for disbudding and dehorning, and reduces the probability of animal injury during (and because of) those procedures.