Alter is genetics so it dosen't produce them.

With seedless seeds.

They grow from seeds like most things. They only difference is they don't reproduce so they don't have seeds. They are produced by treating a variety of seeds with a chemical which alters the chromosomes into doubling their number. Doubling a normal (diploid) watermelon results in a tetraploid plant (one having four sets of chromosomes). When the tetraploid plant is bred back, or pollinated, by a diploid or normal plant, the resulting seed produces a triploid plant that is basically a "mule" of the plant kingdom, and it produces seedless watermelons.

"Seedless fruits can develop in one of two ways: either the fruit develops without any fertilization (parthenocarpy), or pollination triggers fruit development but the ovules or embryos abort without producing mature seeds (stenospermocarpy). Seedless fruits of banana and watermelon are produced on triploid plants, whose three sets of chromosomes prevent meiosis from taking place and thus do not produce fertile gametes. Such plants can arise by spontaneous mutation or by hybridization between diploid and tetraploid individuals of the same or different species. Some species, such as pineapple and cucumber, produce seedless fruit if not pollinated, but produce seeded fruit if pollination occurs. Lacking seeds, and therefore the capacity to propagate via the fruit, the plants are generally propagated vegetatively from cuttings, by grafting, or in the case of bananas, from "pups" (offsets). In such cases, the resulting plants are genetically identical clones. By contrast, seedless watermelons are grown from seeds. These seeds are produced by crossing diploid and tetraploid lines of watermelon, with the resulting seeds producing sterile triploid plants. Fruit development is triggered by pollination and these plants must be grown alongside a diploid strain to provide pollen. One disadvantage of most seedless crops is a significant reduction in the amount of genetic diversity in the species. As genetically identical clones, a pest or disease that affects one individual is likely to be capable of affecting every clone of that individual. For example, the vast majority of commercially produced bananas are cloned from a single source, the Cavendish cultivar, and those plants are currently threatened worldwide by a newly discovered fungal disease to which they are highly susceptible." Source and further information: http://en.wikipedia.org/wiki/Seedless_fruit