Ka/Ks Ratio

Ka/Ks Ratio

In genetics, the K_a/K_s ratio (or ω, d_N/d_S), is the ratio of the number of non-synonymous substitutions per non-synonymous site (K_a) to the number of synonymous substitutions per synonymous site (K_s), which can be used as an indicator of selective pressure acting on a protein-coding gene. Comparisons of homologous genes with a high K_a/K_s ratio are usually said to be evolving under positive selection. The terms K_a/K_s and d_N/d_S are used interchangeably. Note however that D_n and D_s are different parameters from d_N and d_S (or K_A and K_S ). D_n and D_s are count estimates, which represent the total numbers of non-synonymous and synonymous substitutions.