Editing Gene expression programming (section)

==Multigenic chromosomes==
The chromosomes of gene expression programming are usually composed of more than one gene of equal length. Each gene codes for a sub-expression tree (sub-ET) or sub-program. Then the sub-ETs can interact with one another in different ways, forming a more complex program. The figure shows an example of a program composed of three sub-ETs.

[[File:Expression of 3 GEP genes, 1st k-expression *Qb+*-bbba.png|thumb|Expression of GEP genes as sub-ETs. a) A three-genic chromosome with the tails shown in bold. b) The sub-ETs encoded by each gene.]]

In the final program the sub-ETs could be linked by addition or some other function, as there are no restrictions to the kind of linking function one might choose. Some examples of more complex linkers include taking the average, the median, the midrange, thresholding their sum to make a binomial classification, applying the sigmoid function to compute a probability, and so on. These linking functions are usually chosen a priori for each problem, but they can also be evolved elegantly and efficiently by the [[gene expression programming#Cells and code reuse|cellular system]]<ref>{{cite book|last=Ferreira|first=C.|title=Gene Expression Programming: Mathematical Modeling by an Artificial Intelligence|url=http://www.gene-expression-programming.com/GepBook/Introduction.htm|publisher=Angra do Heroismo |location=Portugal|year=2002|isbn=972-95890-5-4}}</ref><ref>{{cite web|last=Ferreira|first=C.|year=2006|title=Automatically Defined Functions in Gene Expression Programming|url= http://www.gene-expression-programming.com/webpapers/Ferreira-GSP2006.pdf|publisher= In N. Nedjah, L. de M. Mourelle, A. Abraham, eds., Genetic Systems Programming: Theory and Experiences, Studies in Computational Intelligence, Vol. 13, pp. 21–56, Springer-Verlag}}</ref> of gene expression programming.