Cistron, recon and muton
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. A
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 and
muton 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, a
cistron of several
recons and a
recon of several
mutons. A
recon and a
muton may be of the same size, so that in such a case
recon may not consist of several
mutons.