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NCERT says that chemical and physical factors which induce mutations are called mutagens, and gives UV radiation as an example. The person who first showed that radiation can change genes, and measured how much, was Hermann Muller. This post covers his life, the experiment, what "proportional to dose" means with worked numbers, common mistakes, and what to learn from his approach.
Who was Hermann Muller?
His Nobel facts page records that he was born in New York on 21 December 1890 and died in Indianapolis on 5 April 1967. He won the 1946 Nobel Prize in Physiology or Medicine "for the discovery of the production of mutations by means of X-ray irradiation".
His Nobel biography says his family had very limited means. He reached Columbia College only through a scholarship awarded on his entrance examination marks, and worked summers as a bank runner and hotel clerk. In 1909 he started a students' biology club whose members included Calvin Bridges and Alfred Sturtevant, and he became part of Thomas Hunt Morgan's fruit fly group. He taught at the Rice Institute in Houston and from 1920 at the University of Texas in Austin, where the X-ray work was done.
What problem was he trying to solve?
Natural mutations are rare and random, which made them nearly impossible to study. From 1918 Muller developed methods to count them, especially lethal mutations, which are easier to detect without bias. In his Nobel lecture he explains that his work with Edgar Altenburg showed that lethal mutations "greatly outnumbered" those with visible effects.
He reasoned that if gentle heat motion inside cells occasionally changed a gene, then the far more energetic hits from X-rays should do so much more often.
What did the 1926 experiment show?
The biography says that in late 1926 he obtained "critical evidence of the abundant production of gene mutations and chromosome changes by X-rays", published in 1927 as Artificial transmutation of the gene in Science.
In the Nobel lecture he summarises the scale: "by a half-hour's treatment", it was possible to get "over a hundred times as many mutations in a group of treated germ cells as would have occurred in them spontaneously in the course of a whole generation." The mutations struck one gene at a time, at random. Radiation also broke chromosomes, and broken ends could rejoin in new orders.
What does "proportional to the dose" mean?
The lecture states that later work by his collaborators "showed definitely that the frequency of the gene mutations is directly and simply proportional to the dose of irradiation", whether X-rays, gamma rays or beta rays were used. Studies down to low doses left, in his words, "no escape from the conclusion that there is no threshold dose".
Worked example: a straight-line dose rule
These are sample numbers for practice, not Muller's data. Suppose a dose D gives lethal mutations in 6 per cent of treated X chromosomes.
Double the dose. Proportional means f = kD. At 2D, f = 2 × 6 = 12 per cent.
Quarter the dose. At D/4, f = 6 ÷ 4 = 1.5 per cent.
One-hundredth of the dose. At D/100, f = 0.06 per cent. Small, but not zero. Among 50,000 treated chromosomes that is 50,000 × 0.0006 = 30 mutations.
Compare with nature (sample). If the natural rate were 0.05 per cent per generation, the dose D would raise it by 6 ÷ 0.05 = 120 times, the same order as Muller's "over a hundred times".
The rule only holds while mutations are rare. At very high frequencies some chromosomes are hit twice and the line bends.
What mistakes do students make about mutagens?
Thinking mutations are aimed. Muller stressed that radiation mutations are accidental, with no link to whether they help the organism.
Assuming most mutations are useful. His work showed most are harmful, many lethal.
Treating all radiation as identical. Both UV and X-rays are mutagens, but the lecture notes that UV, besides causing mutations itself, hinders chromosome breakage or the rejoining of broken pieces.
Confusing gene and chromosome changes. A point mutation alters one gene; breakage and rejoining rearranges chromosome pieces.
NCERT's Principles of Inheritance chapter covers point mutations and mutagens. Our posts on point and frameshift mutations and Alfred Sturtevant, his friend from the fly room, take the story further. If the link between DNA changes and traits feels abstract, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing.
Where did his career take him?
The biography records a year in Berlin in 1932, three and a half years at the Institute of Genetics in Leningrad and Moscow, and then Edinburgh from 1937 to 1940, where many of his graduate students came "largely from India". He moved to Indiana University in 1945. In the lecture he argued that since most mutations are harmful, their random production in people "should so far as possible be rigorously avoided", and that radiologists should insist on shielding the gonads whenever people are exposed to radiation in industry or medicine.
What can you take from how he worked?
Build the measuring tool first. Years of method-building came before the X-ray result.
Reason from mechanism. He predicted radiation would work because he had a picture of how mutations happen.
Follow results to their consequences. He turned a lab finding into a public warning about radiation.
Next step: plot the sample numbers from the worked example on a graph of mutation frequency against dose and check that they fall on a straight line through the origin.
