The gaps or spaces between the genes that lie on chromosomes termed intergenic regions. By definition, intergenic regions are non-coding, (i.e., they are not transcribed into codons) and therefore do not direct the protein synthesis via messenger RNA (mRNA). Although not transcribed, intergenic regions have been shown to play a role in the expression of adjacent genes and there is increasing evidence that they contain important control sequences.

As opposed to often conserved gene sequences, genetic variability increases in intergenic regions. Correspondingly, coding regions of DNA are far less variable than intergenic regions. Such a discrepancy is expected considering that the coding regions are parts of alleles that instruct the synthesis of proteins--and that these proteins in turn regulate cellular structure and function in such a manner that they are more directly influenced by selection pressures.

The physical size of genes, and the related physical...