Question

researcher compares a coding dna sequence in rats to the corresponding sequence in humans and finds 420 non-synonymous sites, 180 synonymous sites, 8 nonsynonymous mu- tations, and 6 synonymous mutations. what is the dn /ds ratio? does this ration suggest that natural selection is acting on this gene? if so, is it positive or negative selection? (a qualitative argument is sufficient.) 4. for the data in the problem above, calculate the fraction of harmful mutations assuming that humans and rats shared their last common ancestor 65 million years ago

Answer 1

The dN/dS ratio calculated as 0.5714 suggests purifying selection is occurring because the rate is less than 1. This indicates non-synonymous mutations are selected against, and most mutations over the 65 million years since humans and rats diverged are likely harmful.

Step-by-step explanation:

The question is about calculating the dN/dS ratio and interpreting it to understand whether natural selection is acting on a specific gene. Firstly, the dN/dS ratio is calculated by dividing the number of non-synonymous mutations per non-synonymous site by the number of synonymous mutations per synonymous site. In this case, dN/dS = (8/420) / (6/180) = 0.5714. A dN/dS ratio of less than 1 suggests that purifying selection (negative selection) is acting on the gene, as non-synonymous mutations are being selected against because they are deleterious.

Regarding the fraction of harmful mutations, the mutation rate for individual genes implies that most mutations are harmful. Given the evolutionary timescale of 65 million years, the majority of the observed mutations between humans and rats are likely deleterious in their effects unless positively selected for due to conferring an advantageous trait. This assumption is further supported by the dN/dS ratio that indicates purifying selection.