Base Modifiers – The Agouti Control Of Black (Bay)

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The Agouti gene is abbreviated as “A” in the dominant form and “a” in the recessive form.  In the dominant form, it controls where the black pigment of black horses (EE or Ee) is seen by restricting the black to the lower legs, the ears, the mane and the tail.  In other words, if a horse has the dominant pairings of the gene (AA, Ae) then the horse will be a bay.  If the horse has the recessive pairings (aa) then the horse will be completely black.  A horse with a red base coat (ee) will not be affected by the bay gene.

Knowing that there are more black horses than red horses due to the dominance of the Black Factor EE, Ee) plus the dominance of the Agouti gene restricting the black to the different parts of the horse (AA, Aa), you can now see why the bay colored horse is so dominant.

  • If you breed a base coat of black (EE, Ee) AA to an AA or an AA to an Aa or an AA to an aa then 100% of the horses will be bay. The resulting pairs will be one of the following: AA, Aa and aA.  Each pair will have a dominant A yielding the basic color of 100% bay on the base coat of black but there will be no black foals.
  • If you breed a base coat of black (EE, Ee)  Aa to an Aa then 75% of the foals will be bay and 25% will be black foals. The resulting pairs will be one of the following: Aa, aA and aa. 3 out of 4 possible results have a dominant A while 1 out of 4 have no dominant A.
  • If you breed a base coat of black (EE, Ee) Aa to an aa then 50% will be bay and 50% will be black. The resulting pairs will be one of the following: Aa and aa.  2 out of 4 possible results have a dominant A while 2 out of 4 have no dominant A.
  • If you breed a base coat of black (EE, Ee) aa to an aa then 100% of the foals will be black. The resulting pairs will all be aa with no dominant A.

The Agouti gene has no effect on the red base color.  Whether the horse is a dark bay or a light “blood” bay is possibly dependent on the number of nucleotides that are in the Agouti gene but I am not clear on the exact mechanism.  However it is NOT because of the red base color coming through.

Some will call a horse that is not a chestnut but has no bay points a “brown” horse but I believe genetically that they are all bay horses.  These “brown” horses will sometimes show their bay points during certain times of the year and length of hair coat.

With the majority of horses expressing the black base coat and the majority of horses expressing the Agouti gene, there is now an understanding of why there is a majority of bay horses over every other color horse before dilutions are taken into account.

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