In Pursuit of the Gene

180 ¨ SEX CHROMOSOMES
first explained it in his paradigm-shifting Science paper of July 11, the eyecolor
factor (red or white) and the X chromosome were “combined, and
have never existed apart.” 59 Later that fall, in a graduate seminar, Morgan
was more explicit, commenting that the red color factor sticks to the X
chromosome and that it may therefore be regarded as being in the X chromosome.
60
In particular, the hypothesis that red was stuck to X accounted perfectly
for the fact that white eyes were found in half of the male grandchildren
and in none of the females. As he explained in his Science paper, when
the white-eyed male was crossed to his red-eyed sisters, the F1 daughters inherited
one red X from their mothers and one white X from their fathers
(see the illustration). The F1 sons, on the other hand, contained a single
copy of the red X chromosome inherited from their mothers. 61 When these
flies were interbred, the F2 females inherited one red X from their red-eyed
fathers and either a red or white X from their mothers. Because red was
dominant over white, all the granddaughters were expected to have red
eyes. On the other hand, the F2 males, which carried only one copy of the X
chromosome, would inherit the red X roughly as often as the white, and
thus half the grandsons would be expected to be white eyed. Of 4,252
grandchildren examined, Morgan found 2,459 red-eyed granddaughters and
1,893 grandsons of whom 1,011 were red-eyed and 782 white-eyed. 62
Meanwhile, Morgan pressed forward with the search for new mutants.
Taking his inspiration once again from de Vries, who had suggested
that radiation might cause mutations in pangenes, Morgan began in May
1910 to subject flies—pupae, larvae, and eggs—to radium rays. 63 In the first
batch of treated flies, one mutant was produced that showed an unusual
pattern of beads along the edge of its wings. When the “beaded mutant”
was bred to his sisters, the mutations reappeared in 1 in 60 flies of the first
generation. By selecting out the beaded types and mating them, Morgan
found he could increase the frequency of the mutation to 1 in 35 in the
third generation and 1 in 12 in the fourth. Finally, after many generations,
he succeeded in isolating a pure-breeding beaded stock. Not only did selection
increase the percentage of abnormal wings, but also it resulted in more
extreme forms.
Although the beaded mutants, which seemed to defy the rules of Men-