There is another article in the lastest Paint Horse Journal on 'The Science of Sabino.' Fascinating read about discovery etc of the sabino gene SB1 and how it works etc and the homozygous form is of course your extreme white horses which are maximally expressed sabino's. According to the article if a horse has two copy's of the SB1 gene homozygous(purebreeding) they will have to have maximally expressed colour. It is caused by a single dominant gene - unlike the tobiano's where it can often be hard to distinguish between homozygous and heterozygous as that gene for colour is polygenic(coming from more than one gene) so you have other modifying factors come into play. The study was actually done on the appearance of all white foals in the Tennesee Walking Horse breed - similar history to the TB with random appearance of white foals from plain ordinary parents.
Taken from the article...
Sabino 1 characteristics
Heterozygous SB1 horses generally exhibit three of the four following characteristics:
- two or more white feet or legs
- a blaze or white patch exending the length of the face
- jagged margins around white areas
- spots or roaning in the midsection
Homozygous SB1 horses are mostly white, at least 90 percent depigmented at birth.
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