Ka/Ks ratio

In genetics, the K_a/K_s ratio, also known as ω or d_N/d_S ratio,^[a] is used to estimate the balance between neutral mutations, purifying selection and beneficial mutations acting on a set of homologous protein-coding genes. It is calculated as the ratio of the number of nonsynonymous substitutions per non-synonymous site (K_a), in a given period of time, to the number of synonymous substitutions per synonymous site (K_s), in the same period. The latter are assumed to be neutral, so that the ratio indicates the net balance between deleterious and beneficial mutations. Values of K_a/K_s significantly above 1 are unlikely to occur without at least some of the mutations being advantageous. If beneficial mutations are assumed to make little contribution, then K_a/K_s estimates the degree of evolutionary constraint.
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