In Pursuit of the Gene

SEX CHROMOSOMES © 179
ation of a full-blown species, but it was a definite and clearly defined mutant
type, which Morgan dubbed speck. In the next generation, he found
that 1 in 15 flies were of the mutant type. This was more like it. Not only
did the strain seem to be jumping, but it was keeping on the jump.
Sometime in May 1910, Morgan spotted a single mutant white-eyed
male fly in a culture filled with normal red-eyed brothers and sisters. When
the white-eyed male was mated to his sisters, 1 of 163 progeny was again
white-eyed. The white-eyed fly seemed to be yet another example of a
spontaneous mutation, and Morgan became more convinced than ever that
the flies had entered a mutating period. He believed he may have finally
found the animal equivalent of De Vries’s Oenothera mutants, and he announced
his new results in an abstract entitled “Hybridization in a Mutating
Period in Drosophila” on May 18, 1910. 56 The mutation for white eyes,
which would change the course of modern genetics, merited only two sentences
tacked on at the end of the abstract.
Although further crosses of the white-eyed male to his red-eyed sisters
continued to yield a small but appreciable number of new white-eyed
“sports,” by June, Morgan’s thinking about “white” had undergone a radical
transformation. 57 If he ignored the additional white-eyed sports, he saw
that the cross of the white-eyed male with his red-eyed sisters behaved like
a standard Mendelian cross of a recessive by a dominant, giving exclusively
red-eyed hybrids in the first generation and the classic 3:1 ratio of red- to
white-eyed when the white-by-red hybrids were interbred. But the truly arresting
thing was that recessive white eyes appeared only among the grandsons,
behaving exactly like color-blindness in humans, which was passed
from the affected male, through his daughters, who were unaffected, to the
grandsons. On June 15 he wrote exuberantly to his friend Goodale from
Woods Hole, “My white eyed fly gives a splendid case of sex limited inheritance:
the F2 gives white eyes only in the males,” 58 and he included a brief
sketch of a model that could be used to explain the results.
As it turned out, it was Morgan, not his flies, who had entered a mutating
period in the summer of 1910. His first great inspiration was that the
sex-limited inheritance pattern of white eye color could be explained if he
assumed that the color factors were linked to the X chromosome. As he