"Colors That Do Not Exist in Purebreds
Purebred Arabians never carry dilution genes. Therefore, purebreds cannot be colors such as dun, cremello, palomino or buckskin. However, there is pictorial evidence from pottery and tombs in Ancient Egypt suggesting that spotting patterns may have existed on ancestral Arabian-type horses in antiquity. Nonetheless, purebred Arabians today do not carry genes for pinto or Leopard complex ('Appaloosa') spotting patterns, except for sabino. ..."
"The leopard complex is a group of genetically-related coat patterns in horses. These patterns range from progressive increases in interspersed white hair similar to graying or roan to distinctive, Dalmatian-like leopard spots on a white coat. Secondary characteristics associated with the leopard complex include a white sclera around the eye, striped hooves and mottled skin. The leopard complex genes are also linked to abnormalities in the eyes and vision. These patterns are most closely identified with the Appaloosa horse breed, though its presence in breeds from Asia to western Europe has indicated that it is due to a very ancient mutation...." Leopard complex
"Spotting or excess white was believed by many breeders to be a mark of impurity until DNA testing for verification of parentage became standard. For a time, horses with belly spots and other white markings deemed excessive were discouraged from registration and excess white was sometimes penalized in the show ring.
To produce horses with some Arabian characteristics but coat colors not found in purebreds, they have to be crossbred with other breeds. Though the purebred Arabian produces a limited range of potential colors, they do not appear to carry any color-based lethal disorders such as the frame overo gene ('O') that can produce lethal white syndrome (LWS). Because purebred Arabians cannot produce LWS foals, Arabian mares were used as a non-affected population in some of the studies seeking the gene that caused the condition in other breeds. Nonetheless, partbred Arabian offspring can, in some cases, carry these genes if the non-Arabian parent was a carrier.
There are six known genetic disorders in Arabian horses. Two are inevitably fatal, two are not inherently fatal but are disabling and usually result in euthanasia of the affected animal; the remaining conditions can usually be treated. Three are thought to be autosomal recessive conditions, which means that the flawed gene is not sex-linked and has to come from both parents for an affected foal to be born; the others currently lack sufficient research data to determine the precise mode of inheritance. Arabians are not the only breed of horse to have problems with inherited diseases; fatal or disabling genetic conditions also exist in many other breeds, including the American Quarter Horse, American Paint Horse, American Saddlebred, Appaloosa, Miniature horse, and Belgian.
Genetic diseases that can occur in purebred Arabians, or in partbreds with Arabian ancestry in both parents, are the following:
- Severe Combined Immunodeficiency (SCID). Recessive disorder, fatal when homozygous, carriers (heterozygotes) show no signs. Similar to the 'bubble boy' condition in humans, an affected foal is born with a complete lack of an immune system, and thus generally dies of an opportunistic infection, usually before the age of three months. There is a DNA test that can detect healthy horses who are carriers of the gene causing SCID, thus testing and careful, planned matings can now eliminate the possibility of an affected foal ever being born.
- Lavender Foal Syndrome (LFS), also called Coat Color Dilution Lethal (CCDL). Recessive disorder, fatal when homozygous, carriers show no signs. The condition has its name because most affected foals are born with a coat color dilution that lightens the tips of the coat hairs, or even the entire hair shaft. Foals with LFS are unable to stand at birth, often have seizures, and are usually euthanized within a few days of birth. In November 2009, Cornell University announced that a DNA test has been developed to detect carriers of LFS. Simultaneously, the University of Pretoria also announced that they had also developed a DNA test.
- Cerebellar abiotrophy (CA or CCA). Recessive disorder, homozygous horses are affected, carriers show no signs. An affected foal is usually born without clinical signs, but at some stage, usually after six weeks of age, develops severe incoordination, a head tremor, wide-legged stance and other symptoms related to the death of the purkinje cells in the cerebellum. Such foals are frequently diagnosed only after they have crashed into a fence or fallen over backwards, and often are misdiagnosed as suffering from a head injury caused by an accident. Severity varies, with some foals having fast onset of severe coordination problems, others showing milder signs. Mildly affected horses can live a full lifespan, but most are euthanized before adulthood because they are so accident-prone as to be dangerous. As of 2008, there is a genetic test that uses DNA markers associated with CA to detect both carriers and affected animals. Clinical signs are distinguishable from other neurological conditions, and a diagnosis of CA can be verified by examining the brain after euthanasia.
- Occipital Atlanto-Axial Malformation (OAAM). This is a condition where the occiput, atlas and axis vertebrae in the neck and at the base of the skull are fused or malformed. Symptoms range from mild incoordination to the paralysis of both front and rear legs. Some affected foals cannot stand to nurse, in others the symptoms may not be seen for several weeks. This is the only cervical spinal cord disease seen in horses less than 1 month of age, and a radiograph can diagnose the condition. There is no genetic test for OAAM, and the hereditary component of this condition is not well researched at present.
- Equine juvenile epilepsy, or Juvenile Idiopathic Epilepsy, sometimes referred to as 'benign' epilepsy, is not usually fatal. Foals appear normal between epileptic seizures, and seizures usually stop occurring between 12 and 18 months. Affected foals may show signs of epilepsy anywhere from two days to six months from birth. Seizures can be treated with traditional anti-seizure medications, which may reduce their severity. Though the condition has been studied since 1985 at the University of California, Davis, the genetic mode of inheritance is unclear, though the cases studied were all of one general bloodline group. Recent research updates suggest that a dominant mode of inheritance is involved in transmission of this trait. One researcher hypothesized that epilepsy may be linked in some fashion to Lavender Foal Syndrome due to the fact that it occurs in similar bloodlines and some horses have produced foals with both conditions.
- Guttural Pouch Tympany (GPT) occurs in horses ranging from birth to 1 year of age and is more common in fillies than in colts. It is thought to be genetic in Arabians, possibly polygenic in inheritance, but more study is needed. Foals are born with a defect that causes the pharyngeal opening of the eustachian tube to act like a one-way valve - air can get in, but it cannot get out. The affected guttural pouch is distended with air and forms a characteristic nonpainful swelling. Breathing is noisy in severely affected animals. Diagnosis is based on clinical signs and radiographic examination of the skull. Medical management with NSAID and antimicrobial therapy can treat upper respiratory tract inflammation. Surgical intervention is needed to correct the malformation of the guttural pouch opening, to provide a route for air in the abnormal guttural pouch to pass to the normal side and be expelled into the pharynx. Foals that are successfully treated may grow up to have fully useful lives.
The Arabian Horse Association in the United States has created a foundation that supports research efforts to uncover the roots of genetic diseases. The organization F.O.A.L. (Fight Off Arabian Lethals) is a clearinghouse for information on these conditions. Additional information is available from the World Arabian Horse Association (WAHO)...."
For More Information:
Five Arabian Horse Female lines: Kuhaylan El Adjus, Siglavy, Habdan, Hamdani and Obajan
Arabian or Arab Horse Breed
Tobiano, Overo, Sabino, Tovero
Molecular Tests for Coat Colours in Horses