In ACAN dwarfism, abnormal cartilage growth leads to skeletal problems and painful dwarfism. Four different disease-causing variants of the ACAN gene are known to date: D1, D2, D3* and D4. All variants show a very similar phenotype and thus the painful dwarfism in Miniature Horses and Shetland Ponies. Affected ponies are often euthanized due to poor quality of life. The disease is inherited autosomal recessively.
Achromatopsia develops as a result of degeneration of the cone cells responsible for color vision. The symptoms are day blindness and impaired vision. The causative mutation for achromatopsia 2 is found in German Shepherds. It is autosomal recessive.
Three base colours occur in the horse: Chestnut (reddish), Black and Bay (Brown).The base colour is determined by the combination of the Agouti Locus (A) and Extension Locus (E).
The Agouti gene (A) controls the ratio of black to red pigment in melanocytes (pigment-producing cells), changing the distribution of black pigment over the body.
One functional gene (AA or Aa) results in the brown base colour (bay), in which the black pigment is concentrated on the legs, mane, and tail. Two non-functional copies (aa) lead to the exclusive production of the black pigment and thus to a black base colour.
The base colour "chestnut" (extension gene "ee") is not influenced by the Agouti gene.
The base colour of the dog is determined by the interaction of the A locus (Agouti), the E locus (Extension) and the K locus (BlacK). The alleles of these loci determine if and where the pigment eumelanin (black colour) and the pigment phaeomelanin (red colour) can be produced and how these two are distributed over the body.
The A locus has several alleles that control the ratio of black to red pigment in the melanocytes (pigment-producing cells) and thus change the distribution of black pigment over the body.
The single genes are in a dominance order. If a higher-ranking (more dominant) allele is present, it also covers existing lower-ranking alleles. Depending on the combination of alleles present, different phenotypes of the dog can occur.
- The Ay allele is inherited dominantly, stands above the others and codes for Fawn/Sable.
- The aw allele is inherited recessively and codes for the Wild Colour ("wolf colour").
- The at allele is inherited recessively and codes for the "Black-and-Tan".
- The a allele is inherited recessively and codes for Recessive Black.
Blue dog syndrome is caused by the dilution mutation (dog's coat, but also the skin, have a dark to light grey colour). This can also lead to severe health problems such as colour dilution alopecia (CDA - dilution colour-related hair loss). This results in a dry, dull and poor coat quality with hair loss and skin lesions increasing with age.