///////// YOUR HORSE’S HEALTH
Two measures are commonly used in determining frequency and characteristics of a genetic disease. Allele frequency is the broad category that covers both dominant and recessive diseases, and is therefore an estimate of the percentage of disease-causing alleles (variants) of a gene among all the alleles in the horse population (in this instance Quarter Horses and Paint horses). Carrier frequency describes the percentage of the population having a single copy of a disease-causing allele that does not cause a disease phenotype (recessive diseases such as LWFS, GBED, and HERDA). These carriers are important because they have the potential to create diseased animals when bred to other carriers, even though they do not show the disease themselves. Breeding two carriers will result in affected offspring 25 percent of the time (and result in carrier offspring 50 percent of the time, and 25 percent chance of a non-carrier). For dominant diseases like HYPP and PSSM, a single allele is enough to cause the disease; breeding an affected animal to a non-affected animal will result in a 50 percent chance of creating a normal animal and 50 percent 68 AUGUST/SEPTEMBER 2017 | REINED COW HORSE NEWS
DR. VALBERG
DR. VALBERG
A muscle biopsy from a PSSM 1 horse.
A muscle biopsy taken from a normal horse.
chance of an affected animal. In the past, many Quarter Horse and Paint breeders did not use DNA testing unless there was a known carrier related to the horses being used for breeding, yet nearly all sub-groups carry these disease alleles. Use of AI, embryo transfer and superovulation and cloning of popular horses can speed up the appearance of genetic diseases and only a few years are needed to pass many copies of a gene to thousands of horses from a popular sire or mare. The need for genetic testing has increased dramatically. “This research paper serves as a reference for occurrence of various genetic diseases in different performance disciplines,” says Winand. “Owners of these horses might think there is low incidence, but it is significant in terms of MH (malignant hyperthermia, a serious genetic disease, which is not included in the panel test now offered by the AQHA) and PSSM—especially the type of PSSM that we still don’t know the cause of. There is a test for type 1 PSSM, but we know there are at least two other types in performance horses such as Quarter Horses, Paints, etc.
“We may not have all the necessary tests yet for these, but if I were breeding for performance horses, I would definitely be testing for PSSM, MH, and GBED since these genetic diseases can occur throughout the population. I wouldn’t worry so much about HYPP and HERDA because these are associated with specific bloodlines and a person would know what to test for and what not to worry about if you know your bloodlines,” she says. Winand adds that breeders have to look back in the pedigree a long ways to be sure, but can understand where these problems came from. By contrast, PSSM and GBED are much more widespread and trace back many generations. “Originally it was thought that GBED came from a mutation passed on by King P234, but essentially all modern Quarter Horses trace back to King at least once, so it could have come from farther back,” Winand explained. In some of her own research concerning the possibilities of cloning being problematic, Winand worked with the AQHA to look at the impact a specific sire could have on the breed.