Agouti

The agouti locus determines to what degree, if at all, the black pigment produced by the E extension allele is restricted and so only has an affect on horses that have the black-base extension phenotype. No restriction results in a horse that is entirely black, but the greater the degree of restriction the less black pigment is actually produced and the more the underlying red-pigment shows through. Many of the various alleles are inactive in donkeys

When creating a custom horse (or using the custom previewer), the user is asked to select the specific genotype at the agouti locus, rather than choosing a phenotype and having the game select an appropriate genotype. There is no correlation between island of origin and agouti allele frequency.

## Alleles

There are four agouti alleles with a simple dominance hierarchy. As a general rule, the more dominant the allele the stronger its restriction ability and the less black pigment is actually displayed on the horse.

### A+

A+ is dominant with respect to A, At and a and has the strongest effect. It can be obtained using the Add A Gene: A+ item. It is inactive in donkeys

### A

A is dominant with respect to At and a but recessive with respect to A+. It can be obtained using the Add A Gene: A item. It is inactive in donkeys

### At

At is dominant with respect to a but recessive with respect to A+ and A. It can be obtained using the Add A Gene: At item.

### a

   a is recessive with respect to A+, A and At and essentially encodes a non-functional version of the agouti gene, exerting no restriction upon black pigment. It can be obtained using the Drop Gene item and selecting 'Agouti'.

## Phenotypes

The following descriptions apply only to horses that have the black-based phenotype, that is at least one E allele at the extension locus. A red-based horse is chestnut regardless of the agouti genotype (though dilution loci may lighten the colour).

### Wild Bay (A+A+, A+A, A+At, A+a)

    Wild bay is the strongest agouti phenotype, representing the most severe restriction of the black-pigment produced by the E extension allele. The black pigment is restricted to the mane, tail, muzzle and lower legs of the horse, with the black extending less up the legs than in the bay phenotype. Black pigment is almost entirely absent from the horse's body and neck. Active alleles at dilution loci may lighten the colour of a wild bay horse. As the A+ allele is inactive in donkeys, the wild bay phenotype is never seen in them and appear black if they have a wild bay genotype.

### Bay (AA, AAt, Aa)

Bay is the second strongest agouti phenotype, reducing the amount of black pigment in the horse's body so that the underlying red shows through. The strength of this reduction is variable, however, and so that a bay horse can appear as anything from light reddish brown as in wild bay to near-black. Regardless, in all cases the mane, tail, muzzle and lower legs are not affected and remain black. Active alleles at dilution loci may lighten the colour of a bay horse. As the A allele is inactive in donkeys, the bay phenotype is never seen in them and appear black if they have a wild bay genotype.

### Brown (AtAt, Ata)

Brown is the weakest active agouti phenotype. The amount of black pigment is reduced in a fairly even fashion to a dark brown that is easily mistaken for black, though the effect is stronger on soft areas of skin such as the muzzle, underarms, belly and groin, making these a lighter brown shade that looks tan in comparison with the dark body. Active alleles at dilution loci may lighten the colour of a brown horse.

### Black (aa)

A black horse is one with only inactive agouti alleles and therefore no restriction affect upon its black pigment. As a result the black pigment is produced throughout the body and the horse is black from head to toe. Active alleles at dilution loci may lighten the colour of a black horse. Donkeys will exhibit this phenotype even if their genetics suggests a wild bay or bay phenotype.