GEP chromosomes are usually composed of more than one gene of equal length. For each problem or run, the number of genes, as well as the length of the head, are a priori chosen. Each gene codes for a sub-ET and the sub-ETs interact with one another forming a more complex multi-subunit ET. The details of such interactions will be fully explained in section 2.2.

Consider, for example, the following chromosome with length 45, composed of three genes (the tails are shown in blue):

It has three ORFs, and each ORF codes for a sub-ET (Figure 1). Position zero marks the start of each gene. The end of each ORF, though, is only evident upon construction of the respective sub-ET. As shown in Figure 1, the first ORF ends at position 8 (sub-ET₁); the second ORF ends at position 2 (sub-ET₂); and the last ORF ends at position 10 (sub-ET₃). Thus, GEP chromosomes contain several ORFs, each ORF coding for a structurally and functionally unique sub-ET. Depending on the problem at hand, these sub-ETs may be selected individually according to their respective fitness (for example, in problems with multiple outputs), or they may form a more complex, multi-subunit ET and be selected according to the fitness of the whole, multi-subunit ET. The patterns of expression and the details of selection will be presented below. However, keep in mind that each sub-ET is both a separate entity and a part of a more complex, hierarchical structure, and, as in all complex systems, the whole is more than the sum of its parts.

Figure 1. Expression of GEP genes as sub-ETs. a) A three-genic chromosome with the tails shown in bold. Position zero marks the start of each gene. b) The sub-ETs codified by each gene.