In the Shorthorn breed, like almost all genetic populations, human and animal, genetic conditions or defects show up from time to time. The terms condition and defect tend to be interchangeable although, generally defect is attached to lethal outcomes.
There are 4 conditions/defects that can be prevalent in Shorthorn populations and testing for them is available through Neogen, the CSA’s Genomic Services provider. Most lethal outcomes can be avoided and exposure can be managed by ensuring that the sire and dam both do not carry the gene and if one does, it is important to be aware of the negative or positive status of the animal it will be mated with.
The CSA’s Digital Beef Registry system provides valuable data on animals in the system by showing whether or not an animal has been tested and if it has, what it’s positive or negative status is.
TH (TIBIAL HEMIMELIA)
TH is a lethal genetic defect when an animal inherits two copies of the recessive gene (sire and dam). Calves are born with severe deformities including twisted back legs with fused joints, large abdominal hernias and/or skull deformities. If calves survive the birthing process, they cannot stand and do not survive long.
In 2006, the CSA made it mandatory that all bulls siring calves in Canada had to be tested for TH before those calves could be registered.
PHA (PULMONARY HYPOPLASIA WITH ANASARCA)
PHA is a lethal genetic mutation when an animal inherits two copies of the recessive gene (sire and dam). Calves are born dead with underdeveloped lungs ( pulmonary hypoplasia) and extreme swelling caused by excessive fluid retention (Anasarca). This is a potentially a doubly lethal defect as the cow often cannot survive trying to birth the calf.
DS (DIGITAL SUBLUXATION)
DS is not a lethal condition from a genetic standpoint. Animals that carry two copies of the undesirable gene (sire & dam) are known as Homozygotes and in the Digital Beef registry carry the symbol ‘DSH” Most cattle that are DSH show some outward signs of the genetic condition, usually malformation of the rear pastern (ankle area). It is important to know that ALL progeny of DSH animals will be at least carriers of the condition and Carriers will be recognized with the symbol DSC in the registry. Although the original mutation happened in completely separate populations, the DS condition sits on the same chromosome as the PHA condition and as a result, it appears that a PHA Carrier animal mated with a DS Carrier can result in the same physical deformities as mating two DS Carriers.
MYOSTATION (ALSO CALLED DOUBLE MUSCLE)
An animal classified as a Myostatin Carrier does not actually have two muscles in place of one normal muscle. However, due to a genetic abnormality, the affected animal has a greater number of muscle fibres which can result in a much larger than normal muscle mass appearance. This condition is common in many mammals and is due to the production of “myostatin”, which is a protein found in the gene that regulates muscle growth. In normal animals, myostatin tells the muscles to stop growing when they reach a certain size however, the defective gene lacks the myostatin protein and therefore allows the muscle to keep growing.
Nine Myostatin mutations (also called variants) of the gene have been identified and some are more common than others in certain Canadian beef breeds. Six of the nine are classified as “disruptive” as they may cause conditions along with extra muscling that are not advantageous such as increased birth weights and calving difficulties. The other three variants are referred to as “missense” or non-disruptive. The nine identified variants are:
E226X, nt419, nt821, C313Y, E291X, 0204X
D128N, F94L, nt821, S105C
The Myostatin condition is not classified as a simple recessive gene defect, like TH for example, where the carrier is an animal having no physical affect, but ‘carries’ the defect (essentially hidden) and can pass it on to offspring. The Myostatin condition affects all that are not “0” for variants. However, that physical affect could be virtually undetectable all the way to obviously affected or to the point of looking deformed. Typically, it is expected that those with only one variant would show much less increased muscularity compared to those with two variants.
In the Digital Beef registry under an animals DNA tab, the test results will typically be presented as in the following examples:
No variants present
1 copy of the variant present
2 copies of the variant present
1, E226X; 1, nt419
1 copy of each of 2 different variants present
Predictable breeding results from mating sires and dams who do and do not have a myostatin variant present in their DNA can be grouped as follows:
0 x 1, V
50% have 0 copies; 50% have 1 copy
1,V x 1,V
25% have 0 copies, 50% have 1 copy; 25% have 2 copies
0 x 2,V
100% have 1 copy
1,V x 2,V
50% have 1 copy; 50% have 2 copies
2,V x 2,V
100% have 2 copies
As with most genetic conditions there are often additional positive and negative associated traits. Positive Myostatin condition characteristics can include: leaner meat (less intramuscular fat), more tenderness, larger rib eye measurement and higher carcass yield. Negative characteristics may include: heavier birth weights, calving difficulties, thinner bones and less fat cover.
For reasons associated with the positive and negative characteristics noted above, breeders may specifically choose to include Myostatin positive animals in their herd and manage them with careful breeding programs. Or breeders may choose to bypass the necessary management and discourage the inclusion of Myostatin positive animals in their herd. Both programs are valid and acceptable and the availability of genetic testing for the presence of the condition ensures management either way is accessible to all members.