breeding brown horses

Teotihuacan SMR 2629 is a seal brown Cerbat stallion

Breeding brown horses together could result in brown, black or chestnut foals (but not bay ones).

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:

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^tA^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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