breeding brown horses

Teotihuacan SMR 2629 is a seal brown Cerbat stallion

If brown is due to a dilution of black not due to agouti then crossing brown horses together would result in black, brown and chestnut foals. There might also be other dilute types depending on whether the dilution gene(s) is/are dominant or recessive, and whether red pigment is affected as well as black (perhaps to make chestnut into sorrel).

If Gowers model of agouti is correct then breeding brown horses together would also result in brown, black or chestnut foals, but not bay ones, as follows below. However the chestnut foals would be expected to be liver and standard chestnut, rather than the chestnut and sorrel which might be more likely for a dilution model.

If both the parents are heterozygous at the extension locus then there is a 1 in 4 (25%) chance of the foal being chestnut. If one or both of the parents are of genotype E⁺E⁺ then only black or brown foals can result. Black foals will only be produced if both parents are of genotype A^tA^a at the agouti locus.

The following demonstrates what happens when two brown horses are bred together which are both heterozygous at both loci:

Genetic contribution from mare:	Genetic contribution from stallion:
Genetic contribution from mare:	E⁺A^t	E⁺A^a	eA^t	eA^a
E⁺A^t	E⁺E⁺A^tA^t brown	E⁺E⁺A^tA^a brown	E⁺eA^tA^t brown	E⁺eA^tA^a brown
E⁺A^a	E⁺E⁺A^tA^a brown	E⁺E⁺A^aA^a black	E⁺eA^tA^a brown	E⁺eA^aA^a black
eA^t	E⁺eA^tA^t brown	E⁺eA^tA^a brown	eeA^tA^t chestnut	eeA^tA^a chestnut
eA^a	E⁺eA^tA^a brown	E⁺eA^aA^a black	eeA^tA^a chestnut	eeA^aA^a chestnut

There is a 9:3:4 ratio of brown: black : chestnut, or put another there is just over 56% chance of a brown foal from any particular breeding. (The agouti allele in the chestnut horses is irrelevant to the phenotype since there is no black pigment to distribute, either uniformly or in the points.)

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