Dexter Dun results from a mutated Tyrosinase Protein 1 (TYRP 1) also called the Brown Locus (b). Dexter dun is the result of a recessive ‘b’ gene on chromosome 8.

• B/B = normal looking black Dexter
• B/b = normal looking black Dexter (carries dun)
• b/b = brown Dexter Dun

Dexter dun is a different dilute mutation than dun found in other breeds of cattle. It is unique to the Irish Dexter breed.

• Dexter Dun affects only black Dexters, making them appear different shades of tan to chocolate color pigment.
• Being recessive, it needs 2 copies (it must be homozygous) in order to be expressed.
• Therefore, every dun colored Dexter will genetically be black (and will carry 1 or 2 black genes), and it will be homozygous for the b (brown) Dexter Dun gene.
• A red Dexter can be heterozygous or homozygous for dilute Dexter Dun, but will be a red cow.

  This painting (above) of a dun cow was painted for David Low by William Shiels in the 1830s,
  to represent Irish Dexter and Kerry cattle when they were still regarded as one breed.

Historically, the Dexter Dun mutation appears to trace as far back as to when Irish Dexter and Kerry cattle were still regarded as one breed (and probably long before that). The b gene responsible for Dun coloration in Dexters was isolated in 2002. [Labs: Genserve of Saskatchewan Research Council, Texas A & M, and UC Davis.]

“The Facts of Dexter Color Genetics”, pdf fact sheet, 05/04/2018 by Kim Newswanger; American Dexter Cattle Association (ADCA).
“How Now, Dun Cow” by Carol Davidson. Paper presented at the Second World Dexter Congress, Australia, 2002. Also on Congress CD.
“The Genetics of Dexter Dun”, a collection of webpages by Michael Trotter and Beverley McCulloch.

» Related Resource: UC Davis Veterinary Genetics Laboratory Cattle Tests
» Related Article: “Genetics of Coat Color in Cattle, a brief review of what is currently known about the genes controlling cattle coat colors and patterns.” By Sheila M. Schmutz, Ph.D., Professor Department of Animal Science College of Agriculture and Bioresources University of Saskatchewan Saskatoon, Canada. This is a major source of information on the internet about bovine coat color.
» Related Article: “Color Patterns in Crossbred Beef Cattle,” Megan Rolf, Oklahoma State University
» Related Article: “White Park Colour Pattern” Research paper presented to the White Galloway conference in Germany in 2014.
» Related Article: “Red Genetics.” Color Genetics explained in all breeds of cattle - featuring the Redliner (red Lowliner)
» Related Article: “Cattle Colour Genetics” A blog studying spotting and hereford marking genetics, by a rancher in Saskatchewan.
» Related Article: “Genetics of Highland Coat Color. In this article I will attempt to explain what is known about the genetics behind coat colour in Highlands and some thoughts we have on what is as yet unproven.” By Glen Hastie, Bairnsley Scottish Highland Cattle, Victoria, Australia. This article also references most major sources of information about cattle color genetics on the internet.
» Related Resource: Bovine Genome.org The Bovine Genome Database project is to support the efforts of bovine genomics researchers by providing data mining, genome navigation and annotation tools for the bovine reference genome. Based on the Hereford cow, L1 Dominette 01449.