If you are talking about the stripe of color on the underside of the tail found on motleys and arabesques, I think this is just because of that type of pattern. I had an Argentine X Colombian (recently sold) that had a fully connected pattern and the same solid color on the underside of his tail. I haven't looked at Colombians (normals) with these traits. But I don't think it signifies a mutation. By the way, my cross also had full stripes down his sides, his whole pattern was common of Argentine boas, and Pete Kahl (the breeder of mine) has had numerous Argentine crosses with the same traits.

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