TH in Shorthorns - A Study Paper
The Shorthorn influence can be found in more than thirty breeds of cattle around the world. Selected for centuries for optimum, dependable performance, the Shorthorn breed has been used as a genetic foundation that ensures increased percentage of calf to cow weights, adequate milk and improved carcass characteristics.
The Shorthorn female is the beef industry’s classic example of optimum. She has become a constant in a global cowherd that is striving to compete for market share in diverse climates and adverse markets while maintaining maternal strengths and improving the end product. The Shorthorn female is the beef industry’s classic example of optimum. She has become a constant in a global cowherd that is striving to compete for market share in diverse climates and adverse markets while maintaining maternal strengths and improving the end product.
Shorthorn bulls are being used in rotational crossbreeding programs throughout the world in operations where high conception rates with minimal maintenance is needed. Shorthorn bulls are aggressive breeders with moderate frames. Some lines of Shorthorn cattle produce smooth-muscled, slender, lightweight calves, which are born easily, gain quickly and have the genetic ability to produce a Choice carcass.
TH and the Shorthorn Breed
While Shorthorn cattle offer a host of competitive advantages, the breed is currently struggling with a genetic defect known as Tibial Hemimelia (TH). As breeding selections are made for desirable characteristics, undesirable traits also appear. This situation is not unique to the Shorthorn breed as genetic defects occur in all breeds of cattle. Shorthorn cattle are no more susceptible to defects than are other breeds. However, some of the more popular pedigrees have been identified as carriers of the defective gene. Pedigrees are essential in the accurate trace back of these inherited defects and rapid identification is essential to curb widespread dissemination of this abnormality.
TH is characterized by severe and lethal deformities in newborn calves. Affected calves are born with twisted rear legs with fused joints, have large abdominal hernias and/or a skull deformity. Should the calf survive the birthing process, they cannot stand to nurse and must be destroyed. Although these calves are a loss for owners, the larger problem for breeders is identifying which normal-appearing cattle carry and pass on the recessive allele.
The American Shorthorn Association (ASA) has been monitoring TH since 1999 when a number of affected calves were submitted to research institutions for scientific review. As more was learned, patterns developed and it was determined that the affected calves had a common ancestral heritage.
These relationships and the phenotypic similarity to a proven genetic disorder in Galloway cattle strongly suggested this was a simple autosomal recessive trait. Past experience with this disorder in Galloway cattle and with other recessive traits suggested numerous carriers could be present in the population.
Other affected calves may have appeared previously but were unreported because they were considered isolated cases, without an apparent hereditary basis. The appearance of multiple cases within a short time period made possible the investigation and discovery of familial relationships revealing the probable hereditary basis. The sudden occurrence of the syndrome in multiple animals within a period of a few months on three different farms could have been due to linebreeding and high inbreeding coefficients in those herds.
In November 2003, the initial ASA Genetic Defect Protocol was approved, then revised in April 2004. The ASA Genetic Defect Protocol outlines the steps Shorthorn breeders should take to report and submit samples on defective calves and their parents for parental verification. The protocol also solicits documentation from attending veterinarians. The protocol made possible the listing of carrier animals.
In contrast, the ASA Genetic Defect Policy, approved in August 2005, outlines policy requirements on AI sires, donor dams, and cloned animals, as well as ASA rights and conditions.
TH and Inheritance
Reasonable medical certainty predicted the TH genetic abnormality was the result of a simple autosomal recessive trait appearing in the homozygous state. As with all genes, recessive traits do not appear unless both recessive alleles are present. The condition was considered the result of a recurrence of a genetic mutation affecting a putative hemimelia locus.
Definitions
- Homozygous - having identical alleles for a single trait.
- Heterozygous - having two different alleles for a single trait.
The inheritance of TH is simple to understand, especially if you already understand how polledness is inherited. TH is inherited in the same way, with the normal allele (N) being dominant to the defective allele (n). A simple recessive mode of inheritance involves only a single pair of alleles, one that is passed on by the sire and one that is passed by the dam. In the polled/horned example, a calf will inherit one allele, either polled (P) or horned (p) from each of its parents. The polled allele is dominant to the horned allele, so both homozygous (PP) and heterozygous (Pp) animals will be polled. Only homozygous recessive (pp) animals will be horned.
Inheritance of the gene for TH works essentially the same way. Both parents pass along one allele, either normal (N) or defective (n) to their offspring. The normal allele is dominant to the defective allele, thus heterozygous (Nn) offspring will be normal appearing, but will carry the defective allele. This allele could then be passed on to their offspring. Only homozygous normal (NN) calves will be TH free.
In the case of TH free animals, all sperm and egg cells produced by these animals possess normal copies of the allele. In contrast, in TH carrier animals, all sperm and egg cells produced by these animals possess a normal copy of the allele or a defective copy of the allele. Since the normal allele is dominant to the defective allele, TH carrier animals do not show symptoms of the disease. Carrier animals produce sperm and eggs which may carry the defective allele. An animal must possess two defective copies of the allele in order to display symptoms.
Matings with an animal that is a TH carrier will result in one of three outcomes:
- The calf will not carry the defective TH allele (NN).
- The calf will appear physically normal, but will be a carrier for the defective TH allele (Nn).
- The calf will have the defective allele (nn) and be afflicted with the characteristics described below:
- Bilateral malformed or absent tibia and abdominal hernia.
- A long shaggy hair coat.
- Retained testicles in males.
- Meningocele -- protrusions of the coverings of the spinal cord or brain.
Managing TH is easy if its mode of inheritance is clearly understood. As explained previously, research indicates TH is controlled by a single pair of alleles, where the normal allele (N) is dominant to the recessive defective allele (n). The following combinations are possible:
Genotype of Animal
NN = TH Free
Nn = TH Carrier
nn = TH Defective
The accompanying chart shows the results of various matings and the inheritance of the defective TH allele. Now a DNA-marker can be used to identify carriers from non-carriers.
Should I use TH carrier sires?
Using TH carrier sires can be risky and potentially expensive. Unless you are sure a TH carrier sire is bred to only TH free cows, there is a chance of producing defective calves. Even if the TH carrier sire is mated only to TH free dams, by definition each calf produced has a 50-50 chance of being a TH carrier.
Shorthorn breeders should not assume progeny out of carrier animals have no value as breeding cattle. Only 50 percent of the progeny of a carrier parent are carriers when mated to a non-carrier. The other 50 percent are TH free.
What should I do with TH carrier or suspect dams?
TH carrier cows have a 50-50 chance of producing carrier offspring when bred to TH free sires. To that extent, their value is potentially diminished. However, it is likely not cost effective in most situations to automatically cull all suspect TH carrier cows. In the case of TH carrier cows being used in embryo transplant, on average, half of their calves will be TH carriers and may be discounted. A smart strategy might be to compromise and cull TH carrier cows and offspring if there are other reasons why they should be culled anyway (i.e. age, pregnancy status, low production, undesirable EPDs, etc.). High producing TH carrier cows may be retained, bred to TH free sires, resulting in some TH free offspring.
Mating Strategies
There are a number of different strategies breeders can use to immediately eliminate the chances of producing any defective calves and ultimately cleanse their herds of the defective gene. Strategies range from:
- Adopting the simple practice of only using sires that are TH free.
- Only breeding TH carriers to TH free animals.
- Culling TH carrier animals from the herd.
Any one of these approaches should greatly reduce the chance of producing a TH defective calf. The strategy which is right for you depends upon the economics of the situation and the extent to which your cowherd may carry the defect. Breeders cannot assume that full-sibs to carrier animals are themselves carriers. They have a 50 percent chance of being TH free.
Reporting Abnormalities
The ASA Bylaws state that it is every member’s duty to promptly report any abnormal condition or genetic defect found in Shorthorn cattle to the association. With the aid of qualified geneticists, every effort will be made to determine the cause and document parentage for every reported defect. Any effort to conceal such information is considered an unethical practice by the ASA and may result in suspension or expulsion from the membership. Report an animal with any abnormality as soon as it is discovered. Full and complete reporting is necessary to provide the documentation necessary to diagnose and evaluate possible corrective actions. Please refer to the ASA Genetic Defect Protocol for requirements and procedures.
Future Implications
The struggle with genetic defects is not over. Not only must diligent testing and breeding decisions be made to eliminate TH, another defect is afflicting Shorthorn cattle. You may have heard reports of “bulldog” calves – many of which were often mistaken for calves afflicted with TH. The new syndrome is not bulldog. Researchers are currently investigating the physical conditions of the defect and have named it PHA (pulmonary hypoplasia with anasarca). This second genetic defect, by all reports, is also affecting other breeds. As more information becomes available it will be passed on to the membership.
ASA Genetic Defect Policy
- Beginning January 1, 2006, all AI sires, donor dams, and cloned animals must be DNA genotyped, parentally verified, and genetic defect tested with results reported to the ASA.
- Beginning January 1, 2006, AI certificates and donor dam certificates will be issued regardless of genetic defect status if genetic defect test results have been reported to the ASA.
- A listing of genetic defect “free” and “carrier” males and females will be maintained on the ASA website if they are parentally verified. This listing will be made available upon request.
- Registration certificates will be issued regardless of genetic defect status.
- ASA reserves the right to conduct random genetic defect testing.
- All cattle consigned to ASA sanctioned sales (NAILE “Heifer Calf Futurity” and NWSS “People’s Choice Bull Sale”) must be tested genetic defect free and parentally verified.
Approved by the ASA Board of Directors (August 5, 2005).
Document dated: August 15, 2005