Genetic information provides positive selection tools
Oklahoma City, Okla. – Although genetic disorders are viewed negatively in the agriculture industry, Dorain Garrick, professor of animal science at Iowa State University, provided producers with a positive perspective on broken genes.
“Producers can look at the glass being half full, or they can look at the glass being half empty,” he said, “but I find it to be good news. Having carriers and knowing about mutations in carriers is good news for selection, not fatal news.”
At the 2013 Beef Improvement Federation Symposium held June 12–15 in Oklahoma City, Okla., Garrick addressed the audience on genetic disorders and prevention.
Embryonic lethal defects
Embryonic lethal defects can cause pregnancies to be aborted before day 56 of gestation, so malformations are not typically seen in progeny. An example of an embryonic lethal defect found in cattle is brachyspina.
Brachyspina, which translates into short spine, is a genetic defect that manifests itself as gross lesions, growth retardation, malformation of the vertebrae and long, slender limbs.
Fewer than one in 100,000 births in the dairy industry are impacted by this mutation. However, because the defect more frequently results in abortion than live births, the actual occurrence of mutation is greater.
When the genetic mutation resulting in brachyspina was associated with over 50,000 groups of genes, researchers determined that seven percent of all Holstein sires were carriers of the defect.
The resulting data suggested that one in 200 matings would be between carriers and one in four of those mating would produce brachyspina calves.
It is expected that more than one in 1,000 births would be affected but, because pregnancy is terminated before birth, brachyspina would have a low disease incidence but high carrier frequency.
“The calves that actually present brachyspina at birth are only a minor fraction of what would be here,” Garrick said. “Many of these potential calves have been aborted about 56 days after insemination.”
Garrick said, on a positive note, embryonic lethal defects require high proportions of carriers before it might be detected from reduced conception rate, increased failure rate or more days to calving.
“These disorders are unlikely to be detected in small herds, or herds using many sires with few offspring per sire,” Garrick explained. “There is also no chance of detection without total herd recording.”
“Errors happen all of the time, and they happen nearly as often as every one in 100 base pairs,” he explained. “The average chromosome has 100 million base pairs so that is a million errors that are made when a chromosome is duplicated – that could be potentially disastrous.”
“There are all kinds of genes that repair those errors, but there are some errors that slip by,” Garrick continues. “Many of these errors will never get transmitted because they do not occur in testicular or ovarian tissue.”
Although genetic errors are inevitable, Garrick said that the severity depends on where on the gene structure the mutation took place.
For example, stop mutations on gene 9111 are potentially damaging.
Mutations on this gene can result in potentially lethal defects such as neural tube defects, growth disorders and porcine stress syndrome. Potentially damaging defects resulting from a stop mutation can manifest as abnormal sperm motility, neonatal stress, impaired heart function and epilepsy.
Managing genetic variation
“One of the ways to avoid this problem is to crossbreed,” Garrick said. “Different breeds are likely to have different mutations. Many of the advancements producers receive from crossbreeding are from minimizing the chance of getting two copies of the mutated genes.”
He also provides outbreeding as a management method or putting up with the defects.
“Culling carrier parents that have outstanding genetics is not recommended,” he said. “If producers have an outstanding animal that carries defects, they have to remember that half that sperm does not carry the defect. What should sensibly be done is test the offspring and cull the ones that are carriers.”
“The focus should not be on eliminating unfavorable alleles and fixing favorable ones, but increasing the frequency of favorable alleles and decreasing the frequency of unfavorable alleles,” he concluded. “This will help to avoid mating that produces homozygous recessive offspring.”