Using a variety of techniques you can create a 'crossover' between 2 genomes to breed the next generation. A simple crossover is to take the first half of one and the second half of the other and attach them together. Or any other way of combining the two to give a 'good' result, but that's the most basic way of crossover before putting the next generation into the growth chamber.
For a better explanation, see Koza's[1] work.