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Taken directly from the 2012 AQHA Rule Book -

205. GENETIC DEFECTS AND UNDESIRABLE TRAITS. In accordance with its Mission Statement and Welfare Policy Statement, AQHA endeavors to educate its membership and protect the welfare and integrity of the breed. Part of such endeavor includes AQHA’s efforts associated with t...he research, identification and promulgation of rules and regulations concerning genetic defects and undesirable traits through AQHA’s governance process. In that regard, researchers have noted that the American Quarter Horse does not have more genetic diseases than other equine breeds, AQHA has just been proactive and taken the lead at identifying genetic diseases affecting the American Quarter Horse breed. The conditions listed below and commonly considered undesirable traits or genetic defects by the Board of Directors shall be indicated on the registration certificate for horses foaled on or after the indicated date, once the condition is known. Upon discovery, the owner shall immediately report such condition to AQHA for marking its condition on the registration certificate as provided below. Failure to timely report these conditions may subject the owner to possible disciplinary action. With respect to the genetic diseases listed below, mandatory testing is only required with respect to HYPP in accordance with Rule 205(c)(2-3). However a panel of DNA tests for all of the genetic diseases listed below (items c through g) is available through AQHA. Subject to Rule 205(c)(3) and/or the rules of the individual event: One or more of these conditions does not prevent a horse from being used as breeding stock or from participating in AQHA-approved events, Genes come in pairs; each parent contributes one copy to the offspring. With respect to the autosomal dominant genetic diseases listed below (HYPP, PSSM, and MH), only one defective gene is necessary to express the genetic disease in question. Such gene can be inherited from either of the parents or from both of the parents.

With respect to autosomal recessive genetic diseases listed below (GBED and HERDA), typically, two copies of an abnormal gene (two copies = a pair) must be present in order for the horse to show signs of the genetic defect in question. Horses with only one copy of the defective gene are considered “carriers”.

(a) Parrot Mouth - either overshot or undershot, defined by the American Association of Equine Practitioners as “no occlusal contact between the upper and lower central incisors.” Designation effective for foals born on or after January 1, 1992.
(b) Cryptorchid - meaning less than two visible testicles descended into the scrotum. Designation effective for foals born on or after January 1, 1992.
(c) Hyperkalemic Periodic Paralysis (HYPP) - designation effective for foals born on or after January 1, 1998. HYPP is an inherited autosomal dominant genetic disease. A muscular disease caused by a hereditary genetic defect that leads to uncontrolled muscle twitching or profound muscle weakness, and in severe cases, may lead to collapse and/or death. According to research, this condition exists in certain descendants of the stallion Impressive, AQHA registration number 0767246.

(1) The following notification shall be placed on registration certificates of foals descending from the stallion Impressive or any other bloodline determined to carry the HYPP gene:
“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.”

When the parent(s) tracing from the HYPP line has tested negative for HYPP with an appropriate designation appearing on their registration certificate, the above notification is not required, and will, instead, be substituted by the designation “N/N”; or, after testing negative for the gene, the notification may be substituted by the designation “N/N” upon request of the owner at his or her expense.

(2) Mandatory testing for HYPP. At such time as AQHA requires mandatory parentage verification of any foals to be registered in either the numbered or appendix registry, (see 202(i)) any foal tracing to bloodlines known to carry the HYPP gene shall be tested for HYPP at the time the genetic testing for parentage is performed. The results will be designated on the registration certificate in lieu of the above notification. Such testing will not be necessary if the foal’s closest ancestors (parents), tracing to the HYPP line, have been tested negative and designated on their registration certificates, these foals will automatically be designated “N/N” on their registration certificate.

(3) Effective with foals born on or after January 1, 2007, all descendants of the stallion Impressive, AQHA registration number 0767246, shall be required to be parentage verified and HYPP tested, subject to the conditions in (c)(2) above. Any foal testing homozygous positive for HYPP (H/H) will not be eligible for registration with AQHA.

(d) Polysaccharide Storage Myopathy (PSSM) is an inherited autosomal dominant genetic disease that causes excess glycogen storage in muscles which can result in tying-up, muscle tremors, and/or gait abnormalities.

(e) Glycogen Branching Enzyme Disease (GBED) is an inherited autosomal recessive disease that terminates protein synthesis which can result in late-term abortions or death in foals shortly after they are born.

(f) Hereditary Equine Regional Dermal Asthenia (HERDA), also known as hyperelastosis cutis (HC), is an inherited autosomal recessive disease that weakens collagen fibers that connect the skin of a horse to the rest of the horse. Affected horses can have fragile hyperextensible skin which can result in tears, scars and lesions. Affected horses are also known to exhibit impaired healing to such injuries.

(g) Malignant Hyperthermia (MH) is an inherited autosomal dominant disease that causes a life-threatening condition that is usually triggered by exposure to certain drugs used for general anesthesia. In susceptible horses, these drugs can induce an uncontrolled increase in skeletal muscle oxidative metabolism which affects the body's capacity to supply oxygen, remove carbon dioxide, and regulate body temperature, potentially leading to circulatory collapse and death if not treated quickly.

(h) White Markings: A horse having white markings with underlying light skin beyond any one of the following described lines shall be eligible for registration by AQHA only if it is parentage verified through DNA typing the offspring, its sire and its dam. Breeders should be aware that the American Quarter Horse, while long recognized, identified and promoted as a solid-colored horse, can and does occasionally produce offspring with overo paint characteristics. Such markings are uncharacteristic of the breed and are considered to be undesirable traits. The following notification shall be placed on registration certificates of horses exceeding these marking limitations:
“This horse has white markings designated under AQHA rules as an undesirable trait and uncharacteristic of the breed.”
(1) A line parallel with the ground drawn around the front leg at the point halfway between the point of the elbow (the center of the olecranon tuberosity or proximal epiphysis of the ulna) and the protrusion on the back of the knee (the accessory carpal bone or lateral styloid process).
(2) A line parallel with the ground at the center of the gaskin on the hind legs. (The center of the gaskin shall be defined as an imaginary point on the front of the gaskin equidistant between the stifle joint and the center of the hock.) The top point of reference to be the bony protrusion on the inside (medial) of the stifle region (technically known as the medial condyle of the tibia) and the most prominent bony protrusion at the top and inside of the hock (technically known as the medial malleolus of the tibia).
(3) A line around the horse’s neck immediately behind the poll and through the midpoint of the throat latch.
(4) Within an area described as two inches on either side of the ventral midline, beginning at a point midway between the front legs and extending to, and including, the sheath and udder. (5) Additionally, there is allowed a single area of white markings with underlying light skin, such that it can be completely covered with a disk one inch in diameter, either free standing on the horse’s body or being a portion of white marking extending past the above prescribed lines.
(6) Areas of white, pink or mottled skin located on the horse’s genitalia, including the sheath or udder, in the axillary region (armpits) or inside the hind legs, including the inner surface of the hindquarters up to and including the ventral surface of the tail, and which are not readily visible when the horse is in a standing position are not considered white markings as described in (h) above.



This page last updated 02/14/12
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