Genetic defects affect all types of livestock. Spider syndrome in sheep and stress syndrome in swine have had significant economic impacts on those populations in the last few decades. In cattle, one of the first major defects studied that had a large rate of occurrence and economic importance on the purebred population was snorter dwarfism in Angus and Herefords in the 1940s-1950s. There have since been other defects appear such as syndactyly, parrot mouth and tibial hemimelia (TH) to name a few. Limousin even had its own breed specific defect with the discovery protoporphyria in the 1990s.
In 2008, the American Angus Association’s announcement of curly calf syndrome (later classified as AM) and subsequently identified defects has brought genetic defects back to the purebred industry’s attention. With new defects being identified annually and tests being developed quickly, it has become a challenge for breed associations to decide how to manage these defects in their herdbooks.
It is important to first understand what these particular defects are and where they originate. All of the major Angus and Red Angus defects of current concern, Arthrogryposis Multiplex (AM), Neuropathic Hydrocephalus (NH), Contractural Arachnodactyly (CA) and Osteopetrosis (OS), are classified as simple recessive deleterious defects, meaning they are caused by a recessive mutation on a single chromosome. A deletion is simply where a segment of DNA is lost from a chromosome. This deletion caused the failure of one of those two genes to produce a protein critical in development of nerve and muscle tissue. So in effect, these defects work exactly like color. A homozygous dominant animal is free of the defect (homo black). A heterozygous animal is a carrier (het. black). A homozygous recessive animal is affected by the defect (red). A carrier shows no ill effects of the defect and will appear completely normal.
It is also important to note that although mating two carriers of a defect together does have a 25 percent chance the resulting calf will be affected by the defect (see illustration); affected calves are rarely seen by producers. This is due to the fact that affected calves are typically aborted or pregnancies are lost early in embryonic development. Due to the abortion of these early pregnancies, some carrier cows were culled for poor reproductive performance without the rancher knowing the reason why. This is also one of the reasons these defects went unnoticed.
To be a carrier or affected by the defect, an animal has to inherit the recessive (defect) gene and it can only inherit the recessive gene if its pedigree traces back to one of the original carriers. The original carriers of these four deleterious mutations are Rito 9J9 of B156 7T26 (AM), GAR Precision 1680 (NH also AM carrier from 9J9), Premier Independence K N (CA) and BUF CRK CHF 824-1658 (OS).
Once these original animals were identified and the tests were commercially available; it became NALF’s decision to develop a policy regarding genetic defect carriers and potential defect carriers. The NALF policy has gone through several evolutions but these are the current requirements.
All potential genetic defect carriers as identified by NALF born after January 1, 2012, must be tested to qualify for registration. Females will be registered regardless of test result. Carrier females will be denoted on their pedigrees and will not be allowed to record embryo transplant progeny. Carrier bulls will not be eligible for registration. Also, any new foundation cow must be tested for six major defects, the four mentioned previously plus TH and PHA.
NALF has struggled with carrier and potential carrier identification for a number of years due to staff reduction and turnover, as well as software issues. So for the majority of 2012, NALF staff has worked to identify carriers by hand. By the time you read this, NALF should either have published or be close to publishing a carrier status list to help breeders identify animals in their herd.
When dealing with carriers or potential carriers, concern is appropriate but panic is not. It is important to breed success, not lose the value of the genetics in these animals when these defects can easily be bred around. This current policy allows you the option to breed around these genetic defects while decreasing the propagation of defects in the population.
Most defect carrier animals in Limousin and Lim-Flex pedigrees are multiple generations back yielding a lower probability of being a carrier. Even if an animal is out of a carrier, it still has a 50 percent chance of being clean of genetic defects. If you use a defect-free bull, then the number of carriers in your herd will decrease by half every generation. By testing, you can keep your current cow herd and clean up your herd through the replacements you keep or buy. Registered carrier cows also make good recipient dams since they cannot pass along that defect to an ET calf and they put those calves into a contemporary group.
Though genetic defects might seem like a major dilemma at first, this issue should be just a bump in the road for a group of breeders that basically eradicated protoporphyria, turned a docility problem into a docility advantage, and moved average yearling scrotal measurements from 29 cm. to over 35 cm. If you, the breeders, approach this issue with the same resolute mind set to make improvements for your customers, the breed will grow from this as it has from every other challenge it has faced.
If you have any questions or problems, call the NALF office or e-mail Brittany Barrick or myself. We’d be glad to help clarify or assist with anything we can.
From all of us in the NALF office, have a Merry Christmas and a Happy New Year! We look forward to seeing at the National Western Stock Show!