Two species of rodents that diverged 15 million years ago have accumulated an estimated 20 nucleotide differences in a gene.Both of these rodents have one generation per year.Suppose sequence analysis of offspring and parents shows that the actual total mutation rate is 1.5 per million generations.Given this information and assuming that all mutations are either deleterious or neutral, approximately _______ of mutations are neutral.

Question

Quizplus · Accepted Answer

To find the proportion of neutral mutations, we first need to estimate the total number of generations that have passed since the two rodent species diverged. Since they have one generation per year, 15 million years ago would correspond to 15 million generations. Given that there are now 20 nucleotide differences between the species, this suggests an average substitution rate of 20/15 million = 1.33 x 10^-6 substitutions per site per generation.
Assuming that all substitutions are either deleterious or neutral, we can estimate the proportion of neutral mutations as the ratio of neutral substitutions to total substitutions. Since the actual total mutation rate is 1.5 per million generations, we would expect 1.5 x 1.33 x 10^-6 = 1.995 x 10^-6 substitutions per site per generation. Of these, approximately 20/2 = 10 substitutions are expected to be neutral (assuming equal rates of neutral and deleterious substitutions). Therefore, the proportion of neutral substitutions is approximately 10/1.995 x 10^-6 = 0.005, or 0.5%.

Two Species of Rodents That Diverged 15 Million Years Ago