Five- Panel Test­ing

Learn de­tails about mod­ern ge­netic test­ing and its pos­i­tive ef­fect on breed­ing de­ci­sions.

Horse & Rider - - September 2016 - By Barb Crabbe, DVM

the Quar­ter Horse world, as well as breeds crossed with Quar­ter Horses. Horses suf­fer­ing mus­cle tremors, weak­ness, col­lapse, and even death made head­lines when the cause was traced to a ge­netic dis­ease at­trib­uted to a prom­i­nent Quar­ter Horse blood­line. When re­searchers an­nounced that HYPP (hy­per­kalemic pe­ri­odic paral­y­sis) linked to one very fa­mous stal­lion named Im­pres­sive, there was an uproar in breed­ing barns across the coun­try.

A decade later breed­ers be­gan to ac­cept the im­por­tance of re­duc­ing the spread of this deadly dis­ease. In 1998, the Amer­i­can Quar­ter Horse As­so­ci­a­tion adopted a rule re­quir­ing all foals de­scend­ing from Im­pres­sive to be tested for HYPP, with re­sults listed on their reg­is­tra­tion papers. In 2007, horses car­ry­ing two genes for this dis­ease were no longer ac­cepted for reg­is­tra­tion. This marked the be­gin­ning of ge­netic test­ing and AQHA’s ef­forts to min­i­mize the spread of ge­netic dis­eases within the breed.

Fast-for­ward to 2016. Ge­netic tests are now avail­able for five dif­fer­ent her­i­ta­ble dis­eases that’ve been iden­ti­fied in Quar­ter Horses and other stock breeds.

Here, I’ll ex­plain what the five-panel ge­netic test is all about. First, I’ll give you a primer on ba­sic ge­net­ics so you can un­der­stand how genes are in­her­ited. Then, I’ll teach you the ba­sics about the five dif­fer­ent dis­eases that make up the fivepanel ge­netic test. With this in­for­ma­tion, you’ll bet­ter un­der­stand how test­ing can help you make in­formed breed­ing de­ci­sions.

Ge­net­ics 101

When egg and sperm get to­gether to cre­ate a foal, there’s a lot go­ing on be­hind the scenes. Size, color, tem­per­a­ment—even some as­pects of health—are pre­de­ter­mined by the way traits passed from the mare and stal­lion com­bine. The science be­hind all of this is called ge­net­ics, and it’s a com­plex, fas­ci­nat­ing process. Here’s how it works.

Ev­ery horse has, within each cell, 32 pairs of chro­mo­somes that con­tain all of the ge­netic in­for­ma­tion that makes him what he is. One set of these pairs came from his dam via the egg, the other from the stal­lion via the sperm. These chro­mo­somes carry more than 30,000 genes, or spe­cific mes­sages, that de­ter­mine dif­fer­ent traits.

Genes can be ei­ther dom­i­nant (they’re ex­pressed even if car­ried on only one set of chro­mo­somes) or re­ces­sive (a match­ing pair must be present to have the trait in ques­tion). Some genes have in­com­plete dom­i­nance, mean­ing the trait is ex­hib­ited most strongly if both sets of genes are present, but can still be present even if there’s only one gene there. Here’s an ex­am­ple of how it works.

Pre­tend that the trait for pa­tience is a dom­i­nant trait—sym­bol­ized by a cap­i­tal P. A small p means your stal­lion (or

mare) doesn’t carry the pa­tience gene. Your stal­lion gets one gene from his dam, and one from his sire. The fol­low­ing com­bi­na­tions can re­sult. Com­bi­na­tion 1: PP. Your stal­lion got a pa­tience gene from each par­ent, so is ho­mozy­gous for pa­tience. Not only will he stand in the crossties for hours with­out com­plaint, but if he’s bred, he’ll pass on a pa­tience gene to his off­spring, too. Com­bi­na­tion 2: Pp. Your stal­lion got a pa­tience gene from one par­ent, but not from the other. He’s het­erozy­gous for pa­tience. Be­cause the gene is dom­i­nant, he’ll still be pa­tient. If the pa­tience gene had in­com­plete dom­i­nance, he’d be pa­tient—just not quite as pa­tient as he would be if he were PP. If he’s bred, only half of his off­spring will get the pa­tience gene from him. The other half will de­pend on their dams to de­ter­mine whether they’ll be pa­tient. Com­bi­na­tion 3: pp. Your stal­lion is ho­mozy­gous re­ces­sive when it comes to the pa­tience gene. He’s not pa­tient at all, and his off­spring will only be pa­tient if they in­herit a pa­tience gene from their dams.

Now let’s change our sce­nario, and pre­tend that the pa­tience gene is re­ces­sive—mean­ing it’s only ex­pressed if both genes are present. In this sit­u­a­tion “p” would stand for pa­tience, and your stal­lion will only be pa­tient with Com­bi­na­tion 3 (pp). Although he’d carry the gene in Com­bi­na­tion 2 (Pp), no one would know he had it, be­cause he wouldn’t be pa­tient at all. Half of his off­spring would in­herit the gene, but they’d only be pa­tient if they in­her­ited an­other pa­tience gene from their dams (mak­ing them pp). And if your stal­lion were PP, there’s no chance any of his off­spring would be pa­tient. Now let’s take a look at the five her­i­ta­ble dis­eases iden­ti­fied in stock breeds and tested for with the five-panel ge­netic test. These in­clude hy­per­kalemic pe­ri­odic paral­y­sis (HYPP), polysac­cha­ride stor­age my­opa­thy (PSSM), glyco­gen branch­ing en­zyme dis­ease (GBED), hered­i­tary equine re­gional der­mal as­the­nia (HERDA), and ma­lig­nant hy­per­ther­mia (MH). This dis­ease causes a dys­func­tion of the chan- nels, or path­ways, that sodium passes through into and out of mus­cle cells. This dis­rupts the con­duc­tion of im­pulses that stim­u­late mus­cle con­trac­tion and can lead to episodes of mus­cle tremors, weak­ness, cramp­ing, and col­lapse. In se­vere cases, an HYPP episode can be fa­tal.

How is it in­her­ited? HYPP is a dom­i­nant gene, with in­com­plete dom­i­nance. This means your horse is likely to be se­verely af­fected if he’s ho­mozy­gous (car­ries two genes for the trait), and less se­verely af­fected if he is het­erozy­gous (car­ries only one gene for the trait). If he’s ho­mozy­gous for the trait, all of his off­spring will have the dis­ease—but its sever­ity will de­pend on whether they in­herit a gene from their dam. If he’s het­erozy­gous, he’ll only pass on the gene to 50 per­cent of his off­spring.

How com­mon is it? While es­ti­mates say that only 1.5 per­cent of Quar­ter Horses carry this gene, its in­ci­dence in hal­ter horse blood­lines is a stag­ger­ing 56 per­cent.

Newspapers in English

Newspapers from USA

© PressReader. All rights reserved.