As we have seen, it was proved through genetic analysis that gene is not a unit of either function or recombination or mutation. Therefore, it became necessary to define the units of function, recombination and mutation. Benzer, in view of his work, coined the terms cistron (unit of function), recon (unit of recombination) and muton (unit of mutation). Cistron was defined as a unit, the elements (alleles) of which exhibit cis-trans phenomenon. It should however, be realized that gene, as a unit of heredity, is still a very useful term and should not be abandoned or substituted by the term cistron. As a matter of fact, cistron is not a synonym for gene, and it is difficult to use this term in practice, since you can not use it unless a cis-trans test has actually been conducted.
At the intragenic level, with the help of recombination studies, micromaps have been prepared for different genes in the same fashion as chromosome maps for different chromosomes were initially prepared at the intergenic level. A gene was thus, further resolved into smaller units called mutational sites. White eye locus and lozenge locus in Drosophila have thus each been resolved into a number of mutational sites. Hundreds of other genes in large number of organisms have similarly been mapped.
It should be realized that at a particular stage of research, the number of mutational sites will be minimum, due to experimental limitations of handling limited population. As experimental techniques are refined, better resolution is possible and the existing mutational sites are further resolved into more sites. As mentioned above, the smallest unit capable of undergoing recombination is called recon.
is further subdivisible into units of mutations called mutons,
and several mutons in a recon
will not be separable due to recombination. Since mutons in a recon
can not be identified due to recombination test, they can be recognized through the changes in amino acid sequence of a polypeptide chain synthesized by, or through the nucleotide sequence of, the cistron
in question. Since a mutation can take place by single base replacement, a single nucleotide pair is the ultimate limit of muton.
Therefore, cistron, recon
are the units, in the descending order of size and since gene is not synonymous to cistron,
as explained above, it will be at the top of this hierarchy. Thus a gene can consist of several cistrons,
of several recons
and a recon
of several mutons.
and a muton
may be of the same size, so that in such a case recon
may not consist of several mutons.