Comic Strips

Allow me to geek out a bit here, for the sake of the curious. Feel free to correct me if you're smarter or more pedantic than me.

Basically, the way seedless watermelons work is that you "breed" two genetically incompatible watermelon varieties together, so that the fruit they bare are seedless. If you took a "seedless" watermelon plant and bred it with a compatible variety, it would produce seeded fruits.

The slightly more detailed version of this: Unlike most animals, plants can have more than one set of chromosomes and be perfectly fine. In fact, having multiple sets of chromosomes can actually be beneficial for plants. Typically plants, like animals, have 2 sets of chromosomes, but sometimes by random chance or otherwise, they can have double the amount (or more). If you cross a plant with a normal amount of chromosomes (2N) with a plant that has double the amount of chromosomes (4N), you get something in between (3N). These 3N plants are often fertile and produce seeds when crossed with other 3N plants, but usually they are not fertile when crossed with 2N or 4N plants. They'll still produce their signature fruit, basically a hormonal response to being pollinated, but there will be no seeds or only underdeveloped, sterile seeds because the parents aren't compatible.