Agouti (A) Gene

  The Agouti Gene, fittingly located at the Agouti Loci (22q15-q16), is controlled by two different alleles: “A” and it’s recessive counterpart “a”. This gene works in cooperation with the Extension (E) Gene, and either allows or inhibits the expression of the Eumelanin pigment produced by the Extension gene. The Agouti gene is composed of a 4,994 base pair sequence containing three exon sequences and two intron sequences.
     The dominant Agouti allele “A” codes for the production of the functional Agouti Signaling Protein (AS1P). This protein, when functional, is the antagonist to the receptor produced by the Extension (E) gene; in other words, AS1P prevents the expression of the black pigment by nullifying the action of alpha-melanocyte-stimulating hormone (a-MSH), produced as a result of the Extension gene. When the allele has no mutations in its exons, it codes for the functional AS1P protein (“A”) which partially nullifies the expression of black pigment, isolating it to the points of the horse and leaving the rest of the body a brown color. 

    The recessive Agouti allele “a” codes for the production of a non-functional Agouti Signaling Protein (AS1P). Mutations can occur in either the introns or exons of the Agouti Gene. These mutations include Single Nucleotide Polymorphisms (SNP’s) which swap one base pair (Adenine, Thymine, Guanine, and Cytosine) for another; and deletions in which a base is removed from the gene. Various mutations in the introns occur, but because introns are removed during the mRNA editing process, these mutations do not affect the final protein. Therefore, the most common mutation at the Agouti Locus that results in the production of a non-functional AS1P is an 11-bp deletion that causes a frame-shift creating a mutated polypeptide sequence. When AS1P is completely non-functional (both alleles have this mutation), the  Extension Gene is not nullified, resulting in complete expression of black Eumelanin pigment.

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