Answer: What an oddity! "Crossing" only affects seed production and hence the subsequent generation of plants, so a cross of the two plants in your garden would not explain your yellow eggplant. There are some varieties that do produce yellow eggplant, and perhaps one of those is included somewhere in the parentage of your hybrid -- occasionally a plant will exhibit an odd branch or "sport" which performs differently from the rest of the plant. Perhaps this is why you have two types of eggplant on one plant.
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