13.
Konopka and Benzer isolated mutant fruit flies that had abnormal daily (circadian)
rhythms. The graph below shows the number of fruitflies that eclose each hour of the day
(eclosion means emergence from the pupal case). Which of these statements can be
reasonably deduced from the given data?
a. The wildtype has a 24 hour rhythm
b. The mutation in Mutant 1 causes lethality
c. The rhythm is abolished in Mutant 2
d. Mutant 2 and Mutant 3 have mutations on different protein-coding genes
The correct statement is option a: “The wildtype has a 24‑hour rhythm.”
Introduction
Konopka and Benzer’s classic experiment on Drosophila circadian rhythms identified clock mutants with altered timing of adult eclosion, revealing genetic control of daily biological rhythms. In entrance exams like CSIR NET, a common question uses their eclosion graphs to test understanding of wild‑type 24‑hour rhythm and mutant phenotypes. Correctly reading these graphs is essential for distinguishing rhythmic, arrhythmic, short‑period, and long‑period mutants in Drosophila.
Understanding the given data
In wild‑type flies, the number of emerging adults peaks once every 24 hours, giving sharp daily eclosion peaks that indicate a robust circadian rhythm. Konopka and Benzer showed that three mutant strains display either no rhythm, a short (about 19 h), or a long (about 28 h) circadian period, all controlled by the period (per) gene.
Option a: Wild type has 24‑hour rhythm
The wild‑type eclosion graph shows regularly spaced peaks that repeat every 24 hours, matching the normal environmental light–dark cycle. This 24‑hour periodicity is the defining feature of a functional circadian clock in Drosophila, so statement a is directly supported by the data and is true.
Option b: Mutant 1 causes lethality
Lethality would appear as very few or no flies eclosing at all, but the Mutant 1 graph shows many flies emerging across time, albeit with altered or dispersed rhythm. Konopka and Benzer reported that their clock mutants were otherwise healthy and viable, differing mainly in circadian timing rather than survival, so Mutant 1 is not lethal and option b is false.
Option c: Rhythm abolished in Mutant 2
An abolished rhythm (arrhythmic mutant) would show eclosion spread randomly across all times with no consistent peaks, representing loss of circadian periodicity. In the question’s graph, Mutant 2 still shows discernible peaks separated by a consistent interval (shorter or longer than 24 h), meaning its rhythm is altered in period, not abolished, so option c is false.
Option d: Mutant 2 and 3 in different genes
Konopka and Benzer performed complementation tests and showed that their three classic mutants (arrhythmic, short‑period, long‑period) map to the same period (per) locus on the X chromosome. Therefore different rhythmic phenotypes of Mutant 2 and Mutant 3 do not imply mutations in different protein‑coding genes; they are alleles of the same gene, so option d is false.
