In Pursuit of the Gene

200 ¨ THE FLY ROOM
most extreme cases, he “improved” the grade of the character but not the
frequency of its inheritance. After a year’s worth of selection Morgan managed
to get a truncate stock that yielded about 90 percent truncate offspring,
but the strain simply could not be induced to breed true. By then,
Morgan had had enough, declaring that the inheritance of certain characters
simply could not be explained by the segregation of stable Mendelian
factors.
Such equivocation was entirely unacceptable to Muller and Altenburg,
who were increasingly viewed as fanatical by Morgan, Sturtevant, and
Bridges for their insistence that all inheritance must be chromosomal and
Mendelian. 43 In particular, Muller and Altenburg refused to accept the idea
that the strange and irregular inheritance of truncated wings was caused
by fluctuations of the underlying genes or their contamination by other
alleles.
Support for the stability of the underlying factors was given a powerful
boost by the recent work of Wilhelm Johannsen, who spent a month at Columbia
in the fall of 1911 as part of a several-month cross-country tour
publicizing his new “genotype-phenotype theory.” 44 Following Galton, “the
creator of the study of exact heredity,” as Johannsen wrote in the dedication
of a 1903 monograph on inheritance, Johannsen studied the inheritance
of size of beans, but unlike Galton, who’d conducted his experiments
on beans that were selected from a bushel bought at a local nursery,
Johannsen had been careful to use seeds of known ancestry. In particular,
he made sure to select seeds that were the descendants of a single selffertilizing
plant, or what he called a pure line, that was sure to be homozygous
for all its genes. 45 Although such a plant produced seeds of varying
weights, some heavier and some lighter, Johannsen’s striking finding
was that progeny grown up from the heavier seeds gave rise to seeds of
the same average weight as plants grown up from lighter seeds. Thus
Johannsen concluded that the variability in size in a pure line was due exclusively
to environmental effects—to the position of the bean in the pod,
for example, or the position of the pod on the plant. The variability in “appearance
type” or “phenotype,” as Johannsen dubbed it, was therefore “a
superficial phenomenon” and had to be distinguished from the underlying
unchanging “genotype.” 46 Galton had gone wrong, Johannsen correctly