The way I understand it (I took a course in this 5 years or so ago but might not remember correctly) it's a matter of ease of matching up certain chromosomes. L

The way I understand it (I took a course in this 5 years or so ago but might not remember correctly) it's a matter of ease of matching up certain chromosomes. Like people, plants tend to have chromosomes in multiples (though not necessarily 2). From one generation to the next, for a given set of chromosomes, a parent only passes half on to the child, and this is pretty much random. That means that for a plant breeder it's very undesirable to have parents with genetic diversity: this diversity results in randomness (unpredictability) in the offspring. BUT you don't want offspring that has identical copies for all these chromosomes because those are not strong genotypes (this is where the hybrid vigor comes in, you want plants with diverse chromosomes). The solution that plant breeders have found is that you take parents which have copied chromosomes, but you take 2 different parents and you control who the parents are. That way the offspring gets a mix, but the content of that mix is highly predictable. Some side issues are that the parents are only suitable for breeding (they don't have strong genotypes themselves) and that if you take this offspring and mix them again you get a genetic shuffle which reintroduces unpredictability. This second generation will also perform very poorly, or at least unpredictably. For seed companies you can tell that this is attractive in 2 ways: they can produce quality seed relatively easily, and the farmer always needs to buy the seed that they use for production because the farmer does not have the parent plants and the second generation is unusable.