Question

The chapter mentions that the MHC locus shows evidence for positive selection. How was this determined?

Group of answer choices

-The rate of nonsynonymous mutations is greater than the rate of synonymous mutations

-The rate of nonsynonymous mutations is less than the rate of synonymous mutations

-The rate of nonsynonymous mutations is equal to the rate of synonymous mutations

Answer 1

The MHC locus shows evidence for positive selection because the rate of nonsynonymous mutations is greater than the rate of synonymous mutations.

Step-by-step explanation:

Evidence for positive selection at the MHC locus was determined by comparing the rates of nonsynonymous to synonymous mutations (non-synonymous/synonymous substitution rate). If the rate of nonsynonymous mutations is greater than the rate of synonymous mutations (>1), it suggests that nonsynonymous substitutions provide some advantage and are being actively selected for in a population, indicating positive selection. This comparison is known as the rate ratio and different values of the ratio imply different types of selection: a ratio less than 1 indicates purifying selection (deleterious nonsynonymous substitutions), a ratio equal to 1 indicates neutral evolution, and a ratio greater than 1 indicates positive selection (adaptive nonsynonymous substitutions).