BY PAUL WILLIAMS TBTS TECHNICAL OFFICER
TECHTALK JULY 2013DOWNLOAD ARTICLE HERE
A genetic condition is a disease or condition caused by an abnormality in an individual's DNA. These abnormalities can range from a single gene mutation to the addition or loss of an entire chromosome.
Genetic conditions have been identified in most species, including humans, and over 400 genetic conditions have been identified in beef cattle. Approximately one quarter of these are caused by a single gene mutation, making them easy to manage through DNA testing. Historically, genetic conditions were managed by extensive progeny testing or by eradicating all known relatives of the affected animal. This resulted in production losses and the potential loss of superior genetics.
Developments in DNA testing and gene probability technology now allow breeders to more easily manage and identify genetic conditions such that production losses and spread of the mutation can be minimised.
Genetic conditions generally result in:
The symptoms of genetic conditions can be quite nonspecific. When large numbers of calves are dying or being born with similar abnormalities, both environmental and genetic causes are suspected. Consequently, there is a need for the surveillance and reporting of abnormal calves to breed societies, veterinarians or beef technical officers if genetic conditions are to be identified. Photographs and DNA samples from affected animals should be collected to aid in the identification of the condition.
Genetic conditions have different modes of inheritance. Many have a simple recessive inheritance of a single gene mutation making them easy to manage. These single gene recessive genetic conditions result in 3 possible genotypes:
Figure 1 provides an illustration of the possible mating outcomes from different mating combinations for a single gene recessive genetic condition.
Figure 1. Possible mating outcomes for different genetic condition genotypes
As illustrated in the diagram, mating a carrier animal to a free animal will avoid production losses, with 100% of calves being unaffected by the genetic condition. But, 50% of the resulting offspring will still be carriers. Mating carrier to carrier will result in 75% normal calves and 25% affected, with two thirds of the normal calves being carriers.
Developments in DNA technology have resulted in diagnostic tests being available for several single gene recessive genetic conditions.
The ability for beef producers to collect a DNA sample (either hair, semen, blood or tissue) and send it to a DNA lab for testing has significantly assisted the management of genetic conditions.
In addition to the development of DNA tests, a range of gene probability software (such as GeneProb) is now available to estimate the probability that an untested animal is a carrier, based on their pedigree and the known DNA test results for animals within that pedigree.
Results from gene frequency software such as GeneProb identify animals as either being free untested (XXFU) or the percentage chance of being a carrier (XX__%) as illustrated in the example in Figure 2.
Figure 2. The possible genotype results for the genetic condition Arthrogryposis Multiplex (AM). DNA testing reports AMA, AMF and AMC while the GeneProb predictions report AMFU and AM__% (eg. AM25%).
There is no "one size fits all" strategy for managing genetic conditions. Before embarking on a management strategy producers should consider:
Generally, the management of genetic conditions can be broken into two components.
Managing the Incidence of Affected Animals – In simple terms, the incidence of animals affected by the genetic condition can be managed by avoiding mating carrier dams to a carrier sire. This may be relatively easy to manage in seedstock herds, but can be extremely difficult in commercial herds which keep no pedigree records. For this reason it is recommended that commercial herds, or seedstock herds that have done minimal DNA testing, should always use bulls that are tested free.
Managing the Incidence of Carriers – Managing the frequency of carrier animals is important in seedstock operations. Where available, the use of DNA testing to determine if an animal is either free or a carrier for a genetic condition broadens the options for breeders looking to manage or eradicate the mutation. Use of software like Geneprob to calculate the risk of particular untested individuals being carriers enables DNA testing efforts to focus on the most high risk animals.
Breeders should carefully consider and develop a management strategy that will be used to manage each genetic condition.
Some examples of management strategies are as follows:
In all situations, if carrier animals are sold, full disclosure of their genetic condition status should be provided.
For further advice regarding the management of genetic conditions, or to find out which genetic conditions have DNA tests available, contact staff at Southern Beef Technology Services (SBTS) or Tropical Beef Technology Services (TBTS).