Slide 9 of 18.

Typically crossover is defined so that two individuals (the parents) combine to produce two more individuals (the children). But you can define asexual crossover or single-child crossover as well. The primary purpose of the crossover operator is to get genetic material from the previous generation to the subsequent generation.

Notice that arrays may be fixed or variable length. They may also be 2 or 3 dimensional (or more). Also common are order-based arrays in which the sequence is important and nodes cannot be duplicated during the genetic operations. You can use more than one operator during an evolution.