I have been told that a horse homozygous for Tobiano cannot be a carrier of the Lethal White Overo/OLWS gene. Is it true?
This is not true, and completely misleading. The white spotting genes causing Tobiano and OLWS are independent from each other, and can be found combined in the same horse. This also includes the possibility of having a Tobiano homozygote horse carrying the OLWS gene. The OLWS gene occurs in many breeds, and is not always manifested in the carrier’s phenotype (meaning, you often cannot tell visually just by looking at the horse, if it is a carrier). Please note that even “solid” (breeding stock) paints, tobianos/toveros, Quarter Horses, and others can and do test positive for this gene on occasion. If unsure, we strongly suggest you have your horse tested for LWO/OLWS, before breeding.