When a pure red horse and a pure white horse mate the coat color of their offspring is a mixture between red and white What is this an example of?

Horse coat color is controlled on several different gene loci so the idea of pure white or pure red is erroneous.

Red Color is controlled at the Extension locus and Non-red color is controlled at the

Agouti locus. The genetics of a horse at the Extension locus masks the genetics at the Agouti locus in some instances.

Any horse that is ee at the Extension locus is chestnut or sorrel.

Any horse that is Ee or EE is not chestnut. The main available alleles at this locus are a (black), A (bay) and At (brown/black bay).

Note that there is no allele for White at either the Extension locus or the Agouti locus.

There are two types of white, Dominant White (there is still a debate on the exsistence of this allele and the original school of thought was that only the Ww white foals were viable and a WW white conceptus died during development) and Maximum expressed Sabino which is SbSb at the

sabino site. Both these genes act as modifiers to the base coat colors. So there really is no "pure".