Inheritance of White markings by Theresa Garton, MD

The series of genes that are responsible for the white markings or lack thereof in boxers, and many other breeds of dogs are as follows: / S - Solid, or self color / si - Irish marking pattern, typical flashy boxer / sp - Piebald, saddle type markings, etc. / sw - extreme white, piebald, color restricted to round tail or head.

Further, there are various "plus" & "minus" factors, that can modify the amount of white in any of these patterns. So, detecting the genotype of any particular phenotype is difficult, and boxers with similar markings may be quite genetically heterogenous.

Boxers are described by Little in his "The Inheritance Of Coat Color In Dogs" as having only the S, si, and sw genes. Some believe that boxers only have the S and sw.

Indeed, the inheritance of white markings, IN THE SHOW POPULATION, is most easily explained by the influence of two genes. The question I raise is which two?

I have been made aware of two litters. One was out of two very plain fawns, the result was two very plain puppies, and two whites. Another litter was sired by a very plain brindle, and out of a white mother. Result: four very plain puppies, and five whites. No flashy markings in either litter, only the two extremes. The plain individuals in question here were from the pet population.

When these results are repeated using plain individuals from the show population, there are completely different results. Plain to plain always gives 100% plain, plain to flashy gives 50% plain and 50% flashy, and plain to white gives 100% flashy.

So, I assert that plain in the pet population and plain in the show population are different genotypes. I think we have been wrong in assuming that our flashy show boxers are Ssw. I think the two genes involved in this population are si and sw, and strong plus factors for more color have resulted in the sisw individual
appearing to have the typical irish marking pattern, also causing the homozygous sisi individual to appear "plain" in our show population.

On the other hand, I think that plain individuals in the pet population have the dominant S gene, which disguises, or is dominant over any of the other marking patterns. So, we could have a plain appearing individual, genetically Ssw, who when bred to another plain appearing Ssw individual produce SS and Ssw individuals, both of which appear plain, and swsw individuals, who are white.

Therefore, I also believe that it is possible to develop a strain of true breeding flashy individuals, who produce a minimum of "plain" markings, and no whites, by breeding two plain individuals from the show population, selecting their flashiest offspring, and continuing on in this fashion. Of course, unless someone else with a different line was doing the same, you would have to start over again every time you did an outcross.

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