My understanding from what I read is that there are alot of horses out there that may show the sabino phenotype but not necessarilly have the SB1 genotype. Also though a maximally expressed true SB1 homozygous horse will throw 100% sabino foals BUT if the dam is not sabino they may only be expressed as minimal sabino as they only have one copy of the gene. But yes there are always other modifiers at work when it comes to white markings but the way the SB1 gene works is that is twice as stong when two copies of the gene are at work compared to just one copy. Hope this makes some sense!
They were actually talking also about the draught horse type sabino - you don't find any white clydies that I have ever heard of and that might be a different sabino type mutation again - NOT the SB1 mutation that is the cause of white TB, Arabs and Tennesee Walking Horses. But that is definately dominantly inherited also. Fascinating stuff
