Alleles for white trim in Australian shepherds (in order of descending dominance): (there are a lot more for other breeds, and sometimes these alleles express themselves differently in other breeds)

Allele	Phenotype	Explanation	Genotype
S	self color	no white trim	SS  - no trim
si	irish spotting	little to moderate white (what is desired)	Ssi - less trim sisi - pure irish spotting pattern Ssp - overly white, body splashes, white going into stifles Ss - extremely white
sp	piebald	overly white extention of irish spotting	sisp - midway between piebald and irish spotting spsp - true piebald siw - very white
s	extreme white	all or nearly all white	ss - almost totally white or white

White develops in fetuses from the underside of the dog out to the limbs, the white on the tail develops on the tip and moves toward the body.

The locus for white is S (self)black, but carry red alleles, making them red-factored.