Equine coat color genetics

Equine coat color genetics determine a horse's coat color. All horses begin genetically with a base coat of "red" (chestnut) or "black.""Genetics of Champagne Coloring." "The Horse" online edition, accessed May 31, 2007 at http://www.thehorse.com/viewarticle.aspx?ID=9686] This base color is designated as "e" for the recessive red allele and "E" for the dominant black allele. Separate genes or "modifiers" act upon the expression of base colors to create all other equine coat colors.

Coat color alleles affect melanin, the pigment or coloring of the coat. There are two chemically distinct types of melanin: phaeomelanin, which is perceived as red to yellow color, and eumelanin, is perceived as brown to black. All coloration genes in mammals affect either the production or distribution of these two chemicals.

Genes appear in sets of two, with each individual gene "option" being called an allele. Because sex cells (sperm and ova) contain only half the usual number of chromosomes, each parent contributes one allele in each gene set to the ensuing offspring. When an individual's gene set contains two copies of the same allele, it is called homozygous for that gene. When it has two different alleles, it is heterozygous. For a recessive gene to be expressed, it must be homozygous, but a dominant gene will be expressed whether it is heterozygous or homozygous. A horse homozygous for a certain allele will always pass it on to their offspring, while a horse that is heterozygous carries two different alleles and can pass on either one.

Background information

There are currently two major theories of equine coat color genetics: those based on the work of the late Dr. Ann T. Bowling of the University of California, Davis and of Dr. Phillip Sponenberg of Virginia Polytechnic Institute. These theories overlap, and have more similarities than differences.

The terms Alleles and Modifiers are used interchangeably in this article. An allele identified with a capital letter is a dominant gene, one identified with a lower-case letter is a recessive gene.

The genetics of white horse markings are not yet fully understood but are probably influenced by multiple alleles.

:"Note: For a quick lesson in genetics and heredity, see the articles on alleles or other footnoted tutorials on inheritance. [http://northonline.sccd.ctc.edu/tstilson/BIO101Inheritance.ppt Online tutorial on heredity]

Alleles and effects

Genetic formulas and color definitions

Lethal roan question

Hintz, H. F. and VanVleck, L. D., published 1979. "Lethal Dominant Roan in Horses". "Journal of Heredity", 70:145-146. This study, based on percentages of roan foals thrown by roan parents, popularized the idea of Lethal Roan Syndrome. The study did not include direct genetic analysis and assumed fetus absorption preventing birth. Similar studies suggested that roan-to-roan breedings were lethal because the resulting embryos were absorbed. This was thought to be true because homozygous roans, horses that carried two alleles for the roan characteristic, were rare.

A recent genetic study of roans by Dr. Bowling refuted Hintz and VanVleck’s inferences. Using genetic analysis, Dr. Bowling found several homozygous roans and no evidence of a Lethal Roan Syndrome. [ [http://www.hancockhorses.com/article-roanQHNews.pdf Untitled-1.indd ] ]

ee also

*Equine coat color
*Horse
*Horse breeding

References

Sources and external links

* [http://www.dreamviewfarm.com/genetics.html DreamView Farm Horse Genetic Color Guide] - Used with author's permission.
* [http://www.vgl.ucdavis.edu/services/coatcolorhorse.php "Horse coat color tests"] from the UC Davis Veterinary Genetics Lab
* [http://www.vgl.ucdavis.edu/services/coatcolor.php "Introduction to Coat Color Genetics"] "from" Veterinary Genetics Laboratory, School of Veterinary Medicine, University of California, Davis. Web Site accessed January 12, 2008
* [http://members.aol.com/MFTHorses/sponenbg.htm Horse color genetics] by Dr. Sponenberg
*D. Phillip Sponenberg, DVM, Ph.D.; Virginia-Maryland Regional College of Veterinary Medicine, Virginia Tech, Blacksburg, VA 24061
* [http://www.hancockhorses.com/article-roanQHNews.pdf "In the Genes."] Quarter Horse News, Dec 15, 2004
* [http://www.equinecolor.com/ Equine color genetics information]
* [http://ultimatehorsesite.com/colors/index.html About Horse Color, Genetics, & Markings] General information on common & rare horse colors, includes photos.