Biochemical Genetics

, Volume 5, Issue 3, pp 231–241 | Cite as

The nonidentity of invariable positions in the cytochromes c of different species

  • Walter M. Fitch


This study answers the question: Are the variable and invariable codons of cytochrome c largely the same in all species? A method is presented for estimating the number of invariable (as opposed to unvaried) codons common to two taxa. The two taxa in this study were comprised of four fungi and four metazoans. Given the number of mutations fixed in each taxon, one calculates the number of codons that would be expected to have fixed mutations in both taxa, in one taxon only, in the other taxon only, and in neither taxon. This expectation depends upon the number of invariable codons that are assumed to be common to both taxa. In the present example, the assumption of 41 invariable codons in common leads to estimates that deviate by less than 2% from the values actually observed. This leads to the conclusion that there are 46 positions that are variable in one taxon but invariable in the other, thereby demonstrating that the invariable codons are not largely the same between the fungi and the metazoans.


Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.


  1. Clarke, B. (1970). Darwinian evolution of proteins. Science 168 1009.Google Scholar
  2. Fitch, W. M., and Margoliash, E. (1967a). A method for estimating the number of invariant amino acid coding positions in a gene using cytochrome c as a model case. Biochem. Genet. 1 65.Google Scholar
  3. Fitch, W. M., and Margoliash, E. (1967b). The construction of phylogenetic trees. A generally applicable method utilizing estimates of the mutation distance obtained from cytochrome c sequences. Science 155 279.Google Scholar
  4. Fitch, W. M., and Markowitz, E. (1970). An improved method for determining codon variability in a gene and its application to the rate of fixations of mutations in evolution. Biochem. Genet. 4 579.Google Scholar
  5. Markowitz, E. (1970). Estimation and testing goodness-of-fit for some models of codon fixation variability. Biochem. Genet. 4 595.Google Scholar

Copyright information

© Plenum Publishing Corporation 1971

Authors and Affiliations

  • Walter M. Fitch
    • 1
  1. 1.Department of Physiological ChemistryUniversity of WisconsinMadison

Personalised recommendations