In Pursuit of the Gene

NOTES TO PAGES 193–194 © 327
23. Ibid.
24. This memory is Sturtevant’s as told to Elof Carlson at California Institute of
Technology in the spring of 1967 (Muller Collection).
25. Muller, “An Episode in Science.”
26. Muller laid out the early history of the development of gene mapping in a letter
to Hugo Iltis (H. J. Muller to Hugo Iltis, Jan. 11, 1951, Muller MSS). He also reviewed
the early history of the Drosophila lab in a monograph he wrote for Erwin
Baur in the late 1920s, which he never completed. This handwritten Baur manuscript
is stored at the Lilly Library (Hermann J. Muller, “Baur Ms.,” 8 folders, Muller
MSS). This history is also discussed by Edgar Altenburg in two letters (Edgar
Altenburg to Jack Schultz, June 12, 1961, and Edgar Altenburg to A. H. Sturtevant,
June 19, 1940, Muller Collection). Muller reviews the history in a letter to
Altenburg (H. J. Muller to Edgar Altenburg, Jan. 6, 1933, Muller MSS).
27. In the effort to explain the coupling and repulsion of alleles, William Bateson
and his longtime associate R. C. Punnett formulated the rococo theory of “reduplication”
in which the ratios of parental to recombination types were thought to occur
in a regular series (Sturtevant, History of Genetics, 40; also see Hermann J. Muller,
“The Mechanism of Crossing-Over. II: The Manner of Occurrence of Crossing-
Over,” American Naturalist 50 [1916]: 287).
28. Using Muller’s distance formula, the distance between two genes was given by
the fraction of single crossovers added to twice the fraction of double crossovers.
29. Two markers are defined to be 1 centimorgan apart if they undergo recombination
in 1 percent of meioses. Muller’s new unit of distance also made it clear that
the frequency of double crossovers was suppressed over shorter distances, as if the
chromosomes had some rigidity that hindered the occurrence of two breaks. To
account for the unexpectedly low number of double crossovers, Muller introduced
the concept of “interference,” which was a measure of the extent to which the first
crossover inhibited a double crossover. He found that there was complete interference
(that is, a complete absence of double crossovers) at distances of 10
centimorgans or less, and interference gradually decreased up to 40 map units,
where it was zero. By restricting himself to only closely spaced triples (in which the
three genes were less than 10 centimorgans apart), Muller showed that “AB + BC
= AC” held exactly across the entire chromosome and thus provided the first rigorous
proof that the genes were arranged in a line.
30. A. H. Sturtevant, “The Linear Arrangement of Six Sex-Linked Factors in
Drosophila, as Shown by Their Mode of Association,” Journal of Experimental Biology 14
(1913): 45.