The cattle industry has faced its share of hard times. From Bovine Spongiform Encephalopathy, commonly known as mad cow disease, to salmonella outbreaks, the past few years have been eventful. One of the latest challenges for this industry is the release of numerous genetic defects.
“More genetic defects are being found now because it is easier to demonstrate a defect is genetic,” said Dorian Garrick, professor of animal science at Iowa State University.
“Between 60 and 100 new mutations occur in every animal, and half of these, along with any historic mutations they inherited from their ancestors, will be passed on to their offspring,” said Tonya Amen, genetic service director for the American Angus Association.
In the Angus breed alone, defects such as Arthrogryposis Multiplex, or curly calf; Contractural Arachnodactyly, or fawn calf; Neuropathic Hydrocephalus, or water head; and Developmental Duplication have been discovered in the past five years. These diseases are caused by loss of function (LOF) alleles. A mutation occurs within these alleles that keeps it from producing the encoded protein.
“We suspect that there are at least 2,000 genes that have LOF alleles floating around in the population,” said Jerry Taylor, professor of genetics and animal sciences at the University of Missouri. “In general, these LOF alleles don’t cause too much of a problem as long as they are low in frequency because the majority of animals will have two functioning copies of the gene or one functioning copy to mask the effect of the LOF allele.”
An animal must be homozygous, or carry two copies of the LOF allele, to exhibit the defect. Being homozygous for the LOF allele is a rare occurrence but a large threat.
“If an animal inherits two copies of the LOF allele, the embryo is unviable and we do not achieve a pregnancy, or the calf is lost during pregnancy or soon after birth,” Taylor said. “In other cases, inheriting two copies of a LOF allele leads to defects, which make the animals unproductive.”
With lost pregnancies and unproductive animals, there is no way for a farmer to make a living, which means farmers do not hold onto many animals that are carriers of the defect.
Blood tests and other technologies have been developed to determine if an animal carries the LOF allele. These tests make it easier to find the animals in the bloodline that might have passed down the LOF and give the producer an accurate number of the animals that must be sold in order to cleanse the herd and dispose of the risk.
“We now have sequencing technologies that allow the discovery of the mutations, which cause defects, in a very short period of time,” Taylor said. “As defects have occurred, the affected animals are being sequenced to identify the causal mutation and then tests can be applied to manage the impact of the defect within a breed.”
These tests are important because they identify the carrier animals so that producers can avoid mating them in hopes to eliminate the defect.
“The biggest issue has been the discovery of disease-causing mutations which are not fully penetrant,” Taylor said. “This means that animals that are homozygous for the disease-causing allele do not all have to have the disease. This causes confusion among breeders and some doubt as to whether the correct disease-causing mutation has been discovered. However, this phenomenon is quite well understood among geneticists and is seen occasionally in humans and dogs.”
There are too many LOF alleles to eliminate the problems through culling or selection, Taylor said.
“What we need to do is sequence the important bulls that have a lot of progeny and identify the alleles that are likely to cause problems,” Taylor said. “Then we need to develop inexpensive genotyping tests that can be used to identify which animals in the population carry these alleles and develop software that can optimize the mating design — which bull gets mated to which cow — to minimize the chance of uncovering a defect.”
There are steps being taken towards this idea, but the majority of producers hear about a defect and decide to stop using the animal completely.
“The fact that an animal carries a defect that we know about does not mean that it has no value or should not be registered or that progeny from the animal should not be registered,” Taylor said. “We just need to manage the issue by genotyping animals that we know are at risk of carrying the defect causing allele, and use carriers in mating only to animals that do not carry the defect causing allele.”