noted on breed registry papers. Other times, people traveling with gray horses who have a pure white hair coat have encountered problems with non-horse-oriented officials such as police officers or border guards who are unclear about a horse who has papers saying it is "gray" when the horse in front of them appears white. To further complicate matters, the skin and eyes may be other colors if influenced by other factors such as white markings, certain white spotting patterns or dilution genes. The genetics of gray The gray gene (G) is an autosomal dominant gene. In simple terms, a horse which has even one copy of the gray allele, regardless of other coat color genes present, will always become gray. This also means that all gray horses must have at least one gray parent. If a gray horse is homozygous (GG), meaning that it has a gray allele from both parents, it will always produce gray offspring no matter the color genetics of the other parent. However, if a gray horse is heterozygous (Gg), meaning it inherits one copy of the recessive gene (g), that animal may produce offspring who are not gray, depending on the genetics of the other parent and Mendelian inheritance principles. Genetic testing is now possible to determine whether a horse is homozygous or heterozygous for gray.<ref>[http://www.animalgenetics.us/Gray.htm "Gray - Horse Coat Color DNA Testing." Animal Genetics, Incorporated.'] . web page accessed August 29, 2008</ref> The gray gene does not affect skin or eye color, so grays typically have dark skin and eyes, as opposed to the unpigmented pink skin of white horses. In 2008, researchers at Uppsala University in Sweden identified the genetic mutation that governs the graying process. The study revealed that all gray horses carry an identical mutation that can be traced back to a common ancestor that lived at least two thousand years ago. The discovery that gray can be linked to a single animal provides an example of how humans have "cherry-picked" attractive mutations in domestic animals. Gray is controlled by a single dominant allele of a gene that regulates specific kinds of stem cells. Homozygous grays turn white faster, are more likely to develop melanomas, and are less prone to develop the "fleabitten" speckling than heterozygous grays. Researchers suggest the pigmented speckles of the “fleabitten” gray, as well as more intense reddish-brown colored areas called “blood” markings, may be caused by