Halting HYPP in Quarter Horses

The American Quarter Horse Association restricts future registration of HYPP gene carriers. By Joanne Meszoly for EQUUS magazine.
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Quarter Horse foals who carry the genes responsible for the potentially debilitating muscle disorder hyperkalemic periodic paralysis (HYPP) will soon be ineligible for registration with the American Quarter Horse Association (AQHA).

Under a rule approved by the AQHA board of directors in March 2004, foals born in 2007 and thereafter who carry two copies of the HYPP gene--known as homozygous positive (H/H)--will be barred from registration. However, foals with only one copy of the HYPP gene (N/H)--heterozygous carriers--will still be registered.

The 2007 deadline allows members to prepare for the new rule, says Gary Griffith, AQHA executive director of registration.

"Many people have already made breeding decisions for the next couple of years and [the board] wanted to give people time," Griffith says, adding that "this has been a controversial subject, but in my opinion, the rule change was widely accepted." A homozygous HYPP-positive horse will pass the gene on to its offspring 100 percent of the time. As of early 2004, Griffith estimates that about 250 H/H positive horses were registered with the AQHA.

A disease that can cause uncontrolled muscle twitching, profound muscle weakness and/or collapse, HYPP is found in some descendants of the popular halter sire Impressive, who was born in 1969. Veterinarians first reported cases of muscle tremors and weakness in related horses in the mid-1980s, but HYPP was not linked to Impressive until 1992, the same year a genetic test for the responsible gene was developed.

Six years later, the AQHA added the following statement to the registration certificates of foals related to Impressive: "This horse has an ancestor known to carry HYPP, designated under AQHA rules as a genetic defect. AQHA recommends testing to confirm presence or absence of this gene."

Subsequently, the AQHA began to require that Impressive descendants who otherwise had to be DNA-tested--for example, foals conceived using frozen semen--undergo testing for the HYPP gene. For all other registrations, however, testing was voluntary.

When the new rule goes into effect, all foals related to Impressive must be HYPP-tested, but Griffith adds, "once a [negative HYPP] line is established, the offspring do not need to be tested."

Griffith estimates that as many as 8,000 to 9,000 Impressive descendants will have to be tested in 2007, but in time that number will drop as negative bloodlines are identified. According to the AQHA, Impressive is one of the top all-time leading halter horse sires, and his line remains prominent in today's halter horse competition.

This article originally appeared in the June 2004 issue of EQUUS magazine.

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