The trick they do with seedless watermelons is really neat.

Basically, they have strain A of watermelons that produces seeds. They take it, and make strain B by doubling up every chromosome. The chromosomal composition of B is the same as A, so the resulting plant behaves the same and still produces seeds. You can then make offspring plants with one A parent and one B parent, taking half the chromosomes from each - so it gets, for instance 15/30 from A and 30/60 from B. The resulting plant is still chromosomally equivalent to A and B, but it has an odd number of chromosomes - 45. So when it goes to create sex cells (which grow into seeds), it can't, because those require that the chromosomes be evenly split. So it grows fruit that are identical to A/B, but that don't produce seeds.