In Pursuit of the Gene

210 ¨ OENOTHERA RECONSIDERED
heritance, was still muddying the waters. Isolated by Morgan in May 1910,
the same month Morgan found the famous white-eyed mutant, beaded was
in many respects the most puzzling of all the several hundred Drosophila
mutants that had been isolated over the subsequent five years. In the early
generations, Morgan had found that beaded parents produced only a small
percentage (from 3 to 10 percent) of beaded progeny, and the strange deformations
of the wings were highly variable in shape and degree. By picking
out the most beaded male and female from each culture to serve as the
parents of the next generation, he had quickly improved both the frequency
and the grade of the beaded stock, and he soon had beaded flies that
produced nearly two-thirds beaded progeny of much more extreme type.
Selection was continued for more than a year, over 25 generations, and just
when he was about to conclude that the system had reached its limit, cultures
were found that were 90 percent pure for beaded, and from these
came a truly pure-breeding stock. But the sudden appearance of a constant-breeding
beaded stock was just as mysterious as the variability that
had been observed in the earlier days, and Morgan concluded that the erratic
behavior of beaded was simply not amenable to conventional Mendelian
explanation. 2
In 1912 the beaded project was given to a short-term member of the
Morgan lab, J. S. Dexter, who used the linkage method just then successfully
worked out by Altenburg and Muller on truncate to identify a chief
gene for beaded on chromosome 3 and an intensifier on chromosome 2. 3
But at the same time that he managed to clarify one aspect of the inheritance
of beaded, Dexter introduced a new complication by showing that
upon crossing with a normal fly, a true-breeding beaded fly gave rise to two
very different classes of progeny, as if it were a heterozygote. The first class,
which consisted of 15 to 30 percent of the progeny, were beaded and the remainder
were not. Furthermore, when the beaded progeny were intermated,
they gave about 25 percent beaded offspring, while the unbeaded
progeny gave only 10 percent, which showed that the difference between
the two classes was genetic. 4
¨ IN 1915, JUST BEFORE leaving for Rice, Muller crossed one of the
true-breeding beaded flies to a normal-winged fly, and selected one normal-