For the past two and a half years, the American Simmental Association has been working with Dr. Jon Beever from the University of Illinois on a specific genetic condition called oculocutaneous hypopigmentation or OH. Animals with OH have uniformly light colored irises coupled with an unusual chocolate coat color. This is not a lethal condition. In fact, the effects of OH seem to be mainly cosmetic. OH is a simple recessive trait meaning an animal must inherit two copies of the mutation to display the trait.
Recently Dr. Beever has found the causative mutation and developed a diagnostic test for OH. Using this diagnostic, an archive of ~245 SimGenetic bulls were tested for OH. The incidence of this particular mutation is very low in the Simmental animals screened to date. The mutation can be traced back to an Angus bull, Sir WMS Warrant, which was likely misdiagnosed as a heterochromia irides HI carrier. Although this mutation possibly originated from the Angus breed, out of over 1,300 Angus animals tested, only one (Sir WMS Warrant) has been identified as a carrier of OH.
Due to the non-lethal nature of this condition and the low frequency of the mutation in the Simmental population, the ASA will not require any testing for the trait. The ASA will add OH to TraitTrac and OH test results will populate the pedigrees similar to other traits. This genetic trait will be treated the same as other largely cosmetic traits like coat color and horned/polled.
Oculocutaneous Hypopigmentation (OH)
Further investigation, including the genotyping of frequently used sires, indicates the mutation is present at a relatively low frequency in the Simmental population. This is consistent with the very low frequency of affected calves reported over the three year period. Examination of carrier pedigrees reveals the Simmental bull, PVF-BF BF26 BLACK JOKER (ASA #1930631), as the most popular recent ancestor with DNA available for testing. However, several of the genotyped carriers do not have this sire in their pedigrees indicating the mutation could be significantly older. Considering this information and the prior description of similar traits in other breeds, namely heterochromia irides (HI) in Angus cattle, the possible origin of this mutation was investigated by obtaining samples from known HI carriers. Although there are very few DNA samples available from these older animals, a sample was obtained for the Angus sire SIR WMS WARRANT (AAA #9196894). Indeed, WARRANT was found to be a carrier of this newly identified mutation. Therefore, it is most likely that the mutation was introduced into the Simmental population by the use of Angus cattle during the development of black purebreds. The subsequent screening of more than 1,200 Angus sires indicates the mutation has most likely been eliminated from the current Angus population via pedigree selection in the early 1980s.
Based on these data, the scientific literature was reviewed in an effort to understand if there were documented features that clearly distinguish between the oculocutaneous hypopigmentation (OH) and heterochromia irides (HI) traits, both of which had been previously described. It is our opinion that the characteristics displayed by these affected Simmental calves is more representative of OH than it is of HI. Additionally, examination of the human and mouse literature also supports this designation. Thus, we suggest that if both phenotypes exist in the cattle population, WARRANT should be designated as an OH carrier. Further screening of current descendants of Angus HI carriers is being conducted but has not identified any additional carriers of this mutation within the Angus population.
Information contained in reports and literature from the 70s and 80s, and in these current Simmental cases, indicate that this abnormal phenotype has little or no effect on the viability or performance of affected individuals. However, in some cases, a possible sensitivity to light has been reported. Thus, we suggest this mutation be monitored similarly to other non-lethal traits such as coat color or horned/polled. As with any recessive condition, breeders can avoid the appearance of affected calves by restricting matings between carrier animals.