Or the whole string can represent a number - this has been used in the basic GA applet . Of course, there are many other ways of encoding. This depends mainly on the solved problem. For example, one can encode directly integer or real numbers, sometimes it is useful to encode some permutations and so on. After we have decided what encoding we will use, we can make a step to crossover.
Crossover selects genes from parent chromosomes and creates a new offspring.
The simplest way how to do this is to choose randomly some crossover point and everything before this point point copy from a first parent and then everything after a crossover point copy from the second parent. There are other ways how to make crossover, for example we can choose more crossover points. Crossover can be rather complicated and very depends on encoding of the encoding of chromosome. Specific crossover made for a specific problem can improve performance of the genetic algorithm. After a crossover is performed, mutation take place.