Questions and Answers
What is ASD and how serious is it in the population of mountain horses?
ASD or Anterior Segment Dysgenesis is a genetic defect of the eye. It basically is a faulty development of the front parts of the eye. However in extreme cases the back parts are also afflicted. ASD is inherited via a semi – dominant gene. In a majority of cases it’s not degenerative – the condition of the eye does NOT deteriorate with time. The defect starts in early embryonic stage. It manifests itself only in horses of chocolate color (horses with a silver dapple gene). The exact connection is not yet known, it is assumed that the gene responsible for ASD is located in a close vicinity of the silver dapple gene on the chromozome. There is no genetic test for ASD gene yet, the only way to detect the presence of ASD in a horse’s eye is an ocular exam performed by a trained veterinarian. Horses that inherited 2 copies of ASD gene can manifest multiple eye abnormalities – cysts, deformities of eye shape, catharacts etc.), however normal vision is rarely impaired even in the case of ASD gene homozygosity (2 copies of ASD gene). In extreme and very rare cases subluxation of the lens and a blindness can occur. In horses with only 1 copy of ASD gene the most common manifestation is presence of cysts while normal vision is retained, which is true for cca 87% of heterozygous horses. The remaining 13% procent of ASD gene heterozygots have no physiological changes of the eye although the ASD gene is present. These are called “quiet carriers” and because of this phenomenon breeders must ALWAYS presume the presence of the ASD gene in every horse with a silver dapple gene. Breeders also should remember that the silver dapple gene could be “hiding” in the genotype of horses of other colors (for example silver dapple doesn’t dilute red pigment in chestnuts or is not visible in phenotype of a champagne horse) and try to avoid breeding two horses with silver dapple genes. It is a responsibility of every true breeder to avoid creating ASD gene homozygous horses and implementing healthy breeding practices that will ensure the genetic health of the breed for the future.
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