Pressure from negative selection and the impact of deleterious mutations on polymorphism cause the ratio in the McDonald–Kreitman test to exceed this ratio. For 10 points each:
[10h] Name this ratio that quantifies the variation [emphasize] between species, which indicates purifying selection when it is less than one. This ratio appears in the denominator of the neutrality index.
ANSWER: dN/dS ratio [or Ka/Ks ratio or omega; accept the ratio of non-synonymous mutations to synonymous mutations]
[10e] According to neutral theory, if the rate of synonymous substitutions in the dN/dS ratio is neutral, most substitutions are due not to selection but to this phenomenon, the random chance of allele frequency changes.
ANSWER: genetic drift
[10m] The original formulation of the McDonald–Kreitman test was derived by comparing mutations in this enzyme within multiple species of Drosophila. In humans, an arginine-to-histidine SNP (“snip”) at this enzyme’s position 47 has been linked to early sites of rice cultivation.
ANSWER: alcohol dehydrogenase [or ADH; accept specific alcohol dehydrogenases, such as ADH-3 or ADH1B]
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