Extension locus

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The extension locus (E) is the locus of the gene for the transmembrane protein MC1R, the mutations of which reduce the amount of eumelanin in the hair to varying degrees and at the same time increase the amount of pheomelanin or vice versa.

Genetics and physiology

The extension locus (E) is the gene for the melanocortin receptor 1 ( MC1R ), whose function is also influenced by the agouti signaling peptide (ASIP), which is encoded by the agoutilocus (A).

There is a dominant allele E D , which causes the affected animals to become completely dark ( melanism ) regardless of the agouti locus . The recessive allele e, if it is homozygous, turns the fur on the whole body red-yellow, since no eumelanin but only pheomelanin can be formed. In between there are several alleles that allow the agouti gene to influence, so that a pattern is possible, for example stripes or spots. It is often difficult to determine whether the extension or the agouti locus is responsible for a mutation, as the animals affected by the mutation may look the same on the outside.

horse

In horses, the dominant allele E allows the agoutilocus to influence. A horse with this allele can be black or brown (brown with black mane and black tail) and also be influenced by various lightening genes and piebald genes. If a horse is homozygous for the allele e, there is a fox (completely brown) with the corresponding variations of such color as in equine coat color genetics described

  • Brauner, genotype: EE or Ee

  • Black, genotype: EE or Ee

  • Fuchs, genotype ee

human

In humans, a mutation in the extension locus that renders the associated protein inoperable leads to red hair and pale skin. The eye color is not influenced by the gene.

dog

In domestic dogs , the extension locus is on chromosome CFA5.

Dogs with the wild-type allele E can produce both eumelanin and pheomelanin, while dogs with the recessive e allele (e.g. Golden Retriever , yellow Labrador Retriever, and Irish Setter ) only produce pheomelanin. However, the color of the red Chow Chow is caused by another, unknown mutation.

Another allele, E M , which is dominant against E and e, results in the formation of a black face mask.

cat

Norwegian forest cat amber and white

In domestic cats, a mutation in the extension locus is responsible for the amber coat color , which is currently only recognized in the Norwegian forest cat .

chicken

Eight alleles of the extended black locus in chickens are known from classical genetics. In the meantime it has been proven that this locus is identical to the mammalian extension locus.

Blind Höhlensalmer

Brown mutation - A Wild-colored Blinder Höhlensalm
D For the brown mutation of heterozygous Blinder Höhlensalm
G For the brown mutation of homozygous Blinder
Höhlensalm J Albino variant of the Blind Höhlensalmers

In the case of the blind cave almer ( Astyanax mexicanus ), several mutations of the extension locus are known as brown mutations, which have developed in different populations of the fish and lead to brown eyes and lightened skin and scales. These mutations render the receptor protein encoded by the gene inoperable. While some populations of the cave salver have complete albinism of the OCA2 type in addition to the Brown mutation, others are only lightened by one of the Brown mutations.

Individual evidence

  1. ↑ Krista Siebel: Analysis of genetic variants of loci for the coat color and their relationship to the color phenotype and to quantitative performance characteristics in pigs. Inaugural dissertation . Institute for Animal Science at the Humboldt University in Berlin, 2001.
  2. ↑ LS Robbins, JH Nadeau, KR Johnson, MA Kelly, L. Roselli-Rehfuss, E. Baack, KG Mountjoy, RD Cone: Pigmentation phenotypes of variant extension locus alleles result from point mutations that alter MSH receptor function. In: Cell . 72 (6), 1993 Mar 26, pp. 827-834. PMID 8458079
  3. ↑ L. Marklund, MJ Moller, K. Sandberg, L. Andersson: A missense mutation in the gene for melanocyte-stimulating hormone receptor (MC1R) is associated with the chestnut coat color in horses. In: Mamm Genome. 7 (12), 1996 Dec, pp. 895-899. PMID 8995760
  4. ^ A b Joshua B. Gross, Richard Borowsky, Clifford J. Tabin : A Novel Role for Mc1r in the Parallel Evolution of Depigmentation in Independent Populations of the Cavefish Astyanax mexicanus. In: PLoS Genet . 2009 Jan; 5 (1), p. E1000326. Epub 2009 Jan 2nd PMID 19119422
  5. ↑ a b S. M. Schmutz, TG Berryere: Genes affecting coat color and pattern in domestic dogs: a review. In: Anim Genet. 38 (6), 2007 Dec, pp. 539-549. Review. PMID 18052939 .
  6. ↑ JM Newton, AL Wilkie, L. He, SA Jordan, DL Metallinos, NG Holmes, IJ Jackson, GS Barsh: Melanocortin 1 receptor variation in the domestic dog. In: Mamm Genome. 11 (1), 2000 Jan, pp. 24-30. PMID 10602988 .
  7. ^ Sheila Schmutz: Melanistic Mask
  8. ↑ SM dirt, TG Berryere, NM Ellinwood, YES core, GS Barsh: MC1R studies in dogs with melanistic mask or brindle patterns. In: J Hered. 94 (1), 2003 Jan-Feb, pp. 69-73. PMID 12692165 .
  9. ↑ dépistage génétique pour la mutation ambre - Le Norvégien: du climat rude scandinave à la chaleur de l'ambre ( Memento of 19 April 2011 at the Internet Archive )
  10. ^ Leif Andersson: Melanocortin Receptor Variants with Phenotypic Effects in Horse, Pig, and Chicken. In: Annals of the New York Academy of Sciences . 994, 2003, pp. 313-318.