13.
A bacterial gene has two alleles, A and B. A scientist measured bacteria grown under two
different treatments in the laboratory, and she measured proportion of the population that
had allele A, repeating the experiment six times in each case. The results are shown
below. Which of the following statements could you make about the alleles?
a. Allele B is beneficial in treatment 1
b. Allele A is beneficial in treatment 2
c. Allele B is beneficial in both treatments
d. The fate of allele A is more variable than that of allele B

Allele A is beneficial in treatment 2, and neither allele shows a consistent advantage in treatment 1; option b is correct.

Understanding the graph and question

A bacterial gene has two alleles, A and B, and the scientist starts both treatments with populations where allele A is at 50% frequency. In each treatment, she repeats the experiment six times and plots how the frequency of allele A changes over time in each replicate. In the graph for treatment 1, some lines go up toward 100% allele A while others go down toward 0%, indicating that A sometimes increases and sometimes decreases. In the graph for treatment 2, all six lines rise from 50% toward higher values (close to fixation), showing a consistent increase of allele A in every replicate.

From population genetics, if an allele repeatedly increases in frequency across independent replicates, it is inferred to have a selective advantage under that environmental condition. If its frequency sometimes increases and sometimes decreases starting from the same initial value, then either selection is weak or absent, and drift plays a major role, so no clear advantage can be assigned to that allele.

Option‑wise explanation

Option a: “Allele B is beneficial in treatment 1”

In treatment 1, the outcome lines for allele A split: in some replicates A goes toward 100%, while in others it falls toward 0%. If allele B were truly beneficial in this treatment, allele A should consistently decline across most or all replicates, moving toward loss as B replaces it. Instead, the mixed outcomes suggest that sometimes A wins and sometimes B wins, consistent with drift or very weak selection, not a clear advantage for B. Therefore, the statement that allele B is beneficial in treatment 1 is not supported by the graph and is incorrect.

Option b: “Allele A is beneficial in treatment 2” (correct)

In treatment 2, every line starts at 50% allele A and then rises steadily toward a higher frequency, in some replicates almost reaching fixation. This consistent increase across six independent repeats means that allele A is favored by natural selection under treatment 2: it confers higher fitness than allele B, so its frequency rises predictably. Random genetic drift alone would not push A upward in all replicates from the same starting frequency; one would expect some lines to go down as well. Thus, allele A is beneficial (has a selective advantage) in treatment 2, making option b the correct answer.

Option c: “Allele B is beneficial in both treatments”

If allele B were beneficial in both treatments, allele A’s frequency would systematically decline over time in both panels, approaching 0% in most replicates. The graphs show the exact opposite pattern in treatment 2, where allele A consistently increases, and a mixed pattern in treatment 1. Because A rises in treatment 2 and does not show a consistent decline in treatment 1, there is no evidence that B is beneficial in either treatment, so this option is clearly wrong.

Option d: “The fate of allele A is more variable than that of allele B”

The plots only show trajectories of allele A; there is no direct plot for allele B, whose frequency is simply 1 – frequency of A at any time. The “fate” (trajectory) of B is therefore perfectly anti‑correlated with that of A: whenever A goes up, B goes down by the same amount. In treatment 1, A shows high variability among replicates, but B must show exactly the same variability; in treatment 2, A consistently increases and B consistently decreases, again with matching variability. Hence, it is not meaningful to say A is “more variable” than B, and this option is incorrect.

Short conceptual recap for exams

• Repeated, consistent increase of an allele’s frequency across replicates from the same starting value indicates directional selection in its favor under that environmental condition.

• Mixed outcomes among replicates from the same start point suggest that drift or weak selection dominates, so no clear fitness advantage can be assigned to either allele.

• Because allele frequencies for A and B sum to 1, their variances over time are identical, so one cannot be “more variable” than the other in a simple two‑allele system.