|Genetic Determinations of Colors in Boxers by Theresa Garton, MD - published July 1997 -on Boxer Rescue Mailing List web pages|
of Colors in Boxers
Fawn - All Boxers have a fawn base coat. The brindling pattern, and white color markings are modifiers of this base coat. Dogs appearing fawn do not have the genes for the brindle modifier. Two fawns bred together will always produce 100% fawn puppies (leaving the white markings out of the equation for now).
Brindle - Brindles have a fawn base coat, but also a modifying gene that produces brindle striping. The stripes may be sparse, and far apart, or so large and numerous that the dog appears black, especially when very young, or from a distance. The dog may have one or two genes for the brindle modifier.
If the dog has two genes for brindling (the dog is said to be "homozygous"), then they will always produce only brindle puppies, even if bred to a fawn (again leaving the white markings out of the equation). A dog with one brindling gene, and one non-brindling gene ("heterozygous"), that is bred to a fawn, on average, will produce 50% fawn and 50% brindle. Two brindles who are heterozygous for the brindling gene are bred together, then on average they will produce 25% fawn, and 75% brindle. Furthermore, 1/3 of the brindles (25% of all puppies) will be homozygous brindle, capable of producing only brindle puppies. A heterozygous brindle bred to a homozygous brindle will produce all brindle puppies, half will be homozygous, half heterozygous. And of course, the fawn-homozygous brindle mating would produce 100% brindle heterozygous puppies - brindles capable of producing fawns.
White marking - White markings are controlled by a gene that is totally separate from the base color. White Boxers often have fawn or brindle spots, indicating whether they would have had a fawn or brindle base color, had not their white markings been so extreme.
There are some disagreements about the actual genes involved in white markings in Boxers, but basically, "plain" or almost solid color Boxers, are believed to have either no genes for white markings, or as having genes for the "low end" of the white marking scale. White Boxers are seen as having two genes for the extreme of the usual white marking pattern. Flashy Boxers are seen as having one gene for no white markings (or low end markings), and one gene for extreme white markings.
Two white Boxers will always produce 100% white puppies. Exacted percentages when two flashy Boxers are bred together are 25% plain, 50% flashy, and 25% white. When two plain Boxers from show lines are bred together, they are expected to produce 100% colored (non-white) puppies, though markings may vary.