Rate of change of concomitantly variable codons

Summary

It was previously shown that about 10% of the codons in cytochromec are variable in any one mammalian species and any one point in time and that the positions of theseconcomitantlyvariable codons (covarions) must change as mutations are fixed. Variability implies the existence of an alternative, non-deleterious amino acid that differs by only one nucleotide replacement from the one presently encoded. This work, in addition to obtaining an independent estimate of the number of covarions, investigates the question: What is the likelihood that a cytochromec covarion will lose its variable status as a result of the fixation of a mutation in another covarion? The results show: 1, the number of covarions is in the range of 4 to 10 in agreement with the earlier result of 10 but suggesting the variability may be even more circumscribed than originally thought; and 2, the likelihood of a covarion loosing its variable status as a result of fixations elsewhere in the gene may be greater than 0.75, suggesting a high turnover rate among the covarions.