Question

Researcher has obtained the following counts of mutations by comparing dna sequences from two different species (between), and by comparing a set of dna sequences sampled from different individuals within a species:

within between
nonsynonymous 12 24
synonymous 16 8

use these data to perform a macdonald-kreitman test. which of the following factors might explain the result?
(1) positive selection
(2) negative selection
(3) balancing selection
(4) selective neutrality

Answer 1

To perform a MacDonald-Kreitman test, calculate the nonsynonymous/synonymous ratios for within-species and between-species comparisons. The ratio of 0.75 within-species and 3 between-species suggests positive selection as the likely factor explaining the result. The correct option is (1).

Step-by-step explanation:

In order to perform a MacDonald-Kreitman test using the provided data, we need to calculate the ratios of non-synonymous to synonymous substitutions for both within-species and between-species comparisons.

Within-species:

Nonsynonymous/synonymous ratio = 12/16 = 0.75

Between-species:

Nonsynonymous/synonymous ratio = 24/8 = 3

To interpret the results, we compare the nonsynonymous/synonymous ratios. If the ratio is less than 1, it suggests purifying (negative) selection. If the ratio is equal to 1, it suggests neutral evolution. If the ratio is greater than 1, it suggests positive selection.

From the provided data, the within-species ratio is 0.75 and the between-species ratio is 3. This indicates a higher rate of nonsynonymous substitutions between species, suggesting positive selection as the likely factor explaining the result. Therefore, the correct answer among the given options is (1) positive selection.