The Basics of Equine Color Genetics
By Triple Crown Intern Rachel Voelker
Equine color genetics can be tricky to understand, but here are some basics that can help you grasp the idea.
- Gene – a unit of heredity that in equine color genetics determines the color of the offspring. Horses have a gene for Extension, Agouti, and one for every other modifier or pattern they might have.
- Allele – each gene has two alleles and only one allele can be passed to the offspring
- Genotype – the genetic code that controls the appearance of the horse (ex. E/e)
- Phenotype – the physical appearance of the horse
- Dominant – a dominant allele will always cover up a recessive allele, it only takes one dominant allele to express the color, it will always be expressed with only a few exceptions, it is represented by a capital letter (ex. E)
- Recessive – an allele that will always be covered up by a dominant allele if present, is represented by a lower case letter (ex. e)
- Homozygous – two of the same alleles at one gene (ex. E/E or e/e), can be homozygous dominant or homozygous recessive
- Heterozygous – one dominant allele and one recessive allele on the same gene (ex. E/e)
The Punnett square is an easy trick to do to quickly figure out the probability of the color of the offspring. In this example, the sire genotype is (E/e) and the dam genotype is (e/e). Therefore, the sire is black and heterozygous at Extension. The dam is red and is homozygous recessive at Extension. Extension is the gene that controls the base color of the horse – red (e) or black (E). The sire genotype is placed on top of the Punnett square. The dam genotype is placed on the side of the Punnett square. Then you trace the letters to the box where they would overlap. This gives you the possible outcomes for the color of the foal. If you want to skip the Punnett square, there is a site called Animal Color Genetics and their color calculator will give you the possibilities. All you have to do is enter your stallion and mare’s color genetics. http://www.animalgenetics.us/Equine/CCalculator1.asp
|Dam Genotype||e||E/e||e/e||50% chance offspring will be Black (E/e)|
|e||E/e||e/e||50% chance offspring will be Red (e/e)|
A more challenging example is a Gray (black dun) stallion bred to a red mare. This is a cross we made at Copper Ridge Gaited Horses this year. We were blessed with a perfect black dun filly. The stallion is heterzygous black (E/e) , heterozygous gray (G/g) and heterzygous dun (D/d). The mare is homozygous recessive (e/e) for red. The red mare is not dominant for gray or dun so for simplicity, I have placed a (g) for non-gray and a (d) for non-dun in each square to match up with what the stallion has thrown for gray or dun. Normally I would place an (n) for a non-gray or non dun-gene, however that would be slightly confusing when the offsrping is non-gray and non-dun.
Missouri Fox Trotter Stallion: Rebel Storm
Missouri Fox Trotter Mare: C.C.’s Yankee Sensation
|Probability of Outcomes||Genotypes||Color Outcomes|
|12.50%||E/e, G/g, D/d||Gray (Black Dun)|
|12.50%||E/e, G/g||Gray (Black)|
|12.50%||E/e, D/d||Black Dun|
|12.50%||e/e, G/g, D/d||Gray (Red Dun)|
|12.50%||e/e, G/g||Gray (Red)|
|12.50%||e/e, D/d||Red Dun|
The outcome: Missouri Fox Trotter Black Dun Filly
Other great sources for a better explanation of equine color genetics are UC Davis, Animal Genetics, and theHorse.com. UC-Davis is a great lab for color testing.