It has happened a few times now but most turn out to be the big sweet yellow. Once in a while the yellow grumi flowers at the same as the rootstock branch I let grow, The rootstock is just the common old trashy black which has smaller fruit that isn't as sweet but the plant is more vigorous. The yellow has obviously had flowers pollinated by the rootstock and there must be incomplete dominance. A pure yellow probably would have better fruit but it is kinda groovy. The small tree usually fruits in December to March and usually has 2 crops in that time so I have no seeds now.