Inheritance of myotonic discharges in American quarter horses and the relationship to hyperkalemic periodic paralysis.

The research investigated the genetic heritage of myotonic discharges (a condition resulting in muscle stiffness) in American Quarter Horses and evaluated its relationship to a muscular disease called Hyperkalemic Periodic Paralysis (HPP). The results found that the condition seems to be commonly passed down through generations, with evidence pointing towards an autosomal dominant mode of inheritance.

Methodology

• The research utilized Electromyography (EMG), a diagnostic technique that records the electrical activity produced by skeletal muscles, to detect myotonic discharges in American Quarter Horse breeding stock.
• The research was conducted on two brood mare farms where a total of 21 horses were tested. Of the 21, six exhibited myotonic discharges and 15 did not.
• The team closely observed the results of mating horses affected by hyperkalemic periodic paralysis (HPP), a muscle disorder characterized by episodes of painless muscle weakness.

Findings

• Myotonic discharges were present in five out of six horses that belonged to a bloodline previously identified as predisposed to HPP. Out of these five, two had shown clinical signs of HPP.
• Only one horse, out of the 15 that did not belong to this bloodline, presented myotonic discharges.
• When both parents were non-myotonic, all their offspring (F1 generation) also did not show any myotonic discharges.
• When one stallion with HPP and myotonic discharges was mated with eight nonmyotonic mares over six years, half of their offspring (F1 generation) showed myotonic discharges.
• When both parents exhibited myotonic discharges, four out of six offspring were myotonic. Two did not show myotonic discharges, indicating no evidence of the condition being sex-linked.

Conclusion

• The results from the research suggest that myotonic discharges can be inherited in an autosomal dominant manner. This means that an offspring only needs one parent to carry the gene for myotonic discharges for them to potentially inherit the condition.
• The research also proposes that Hyperkalemic Periodic Paralysis and myotonic discharges, although appearing as different conditions, might actually be different symptoms of the same underlying genetic defect.