Ever eaten a seedless watermelon? Rather like a mule is a hybrid of a horse and a donkey, seedless watermelons are the result of crossing two genetically different varieties of watermelon. The hybrid melon has seeds, but when its seeds are planted, the watermelons sprouting from the new plant contain only underdeveloped seeds. The seeds won't grow a new plant. But you can eat a watermelon wedge without having to spit.
When seedless grape varieties are fertilized, the egg cells form underdeveloped seeds. Grape eaters don't notice them sence, unlike functioning seeds, these seeds don't form tough outer coats. When growers need a new grapevines, they just use cutting from the old vines.
Using techniques like cutting, grafting, and cloning, growers can make endless copies of a desirable plant. That way, the same size, shape, and flavor of a fruit can be reproduced year after year.