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Method in population Genetics website

Synonymous and nonsynonymous rate

In genetics, the $$K_a/K_s$$ ratio is an indicator of selective pressure (the force applied by natural selection) acting on a protein-coding gene. It is calculated as the ratio of the number of nonsynonymous substitutions per non-synonymous site (Ka), in a given period of time, to the number of synonymous substitutions per synonymous site (Ks), in the same period.

Homologous genes with a Ka/Ks ratio above 1 are evolving under positive selection, meaning that at least some of the mutations concerned must be advantageous. If all the mutations are neutral or disadvantageous, the ratio will be in the range 0 to 1. However, if some of the mutations are advantageous and some disadvantageous, the ratio could be less than 1.