In Pursuit of the Gene

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B
f
B
f
+ B
B
fu
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B
B
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f +
TRIUMPH OF THE MODERN GENE © 253
Double bar
No bar
Illustration of unequal crossing over, where B stands for and f and fu are two
recessive mutations that are used to track the crossover types. Illustration by Tamara L. Clark.
crossing over, except that it is between non-homologous regions and at a
time other than the proper synaptic period.” 27
While he accepted Sturtevant’s suggestion that unequal crossing could
take place between nonhomologous regions, Muller categorically rejected
Sturtevant’s idea that normal flies lacked a Bar locus. Instead he proposed
that Bar was caused by a duplication of a small section of the chromosome
containing the normal Bar gene—a “duplication in situ,” he called it—accompanied
by a simultaneous mutation of the new copy. 28 The idea that a
gene was duplicated and then underwent a simultaneous mutation was,
Muller admitted, a weakness of his theory. Nevertheless, he went on to
point out, if Bar were indeed caused by some kind of duplication, then the
unequal crossing over that Sturtevant had suggested to explain the regeneration
of the normal type from Bar would not generate a chromosome
lacking the Bar locus but rather a chromosome containing a single
unmutated copy.
Three years later, in 1936, Muller would revisit the Bar problem, and his
ideas about chromosomes duplications would turn out to provide not only
a complete resolution of the Bar case, but also the key to understanding the
fu
fu