What causes the white coloring is the piebald gene Sp
Yorkies should't carry the piebald gene in the S Locus.
Both parents MUST be SS because the white spotting alleles are thought to be examples of
incomplete dominance.
Meaning that "a heterozygous dog will express its most dominant gene, but may also be affected by the more recessive one to a lesser extent. For example an Ssp dog may have some white spotting"
Dog Coat Colour Genetics
More info you can find here
Goldenray Genetics Parti Yorkies Parti Color and Biewer Colors Genetic Studies
and also here
http://nebula.wsimg.com/058261b970cb...&alloworigin=1 