In Pursuit of the Gene

192 ¨ THE FLY ROOM
near one another in a linear series along the chromosome, adding that “the
proportions that result are not so much the expression of a numerical system
[as Bateson had suggested] as of the relative location of the factors in
the chromosomes.” 21
Sturtevant took the next big step later that year. 22 Half a century later,
he still vividly recalled the afternoon that he hit upon what was without
doubt the greatest idea of his life. He and Morgan had been discussing a
1909 paper by the American geneticist W. E. Castle in which Castle seemed
to be suggesting, contrary to the inspired deductions of the great European
cytologist Wilhelm Roux in 1883 and Morgan’s recent elaboration on the
same theme, that genes were not arranged in a line along the chromosomes
after all. Morgan and Sturtevant agreed that Castle’s idea was not
worth further consideration—the genes were almost certainly arranged in
a line along the chromosomes, and the association between them, as Morgan
had recently pointed out, was a function of their distance apart. It was
then that Sturtevant had his insight, suddenly seeing that the ratio of crossover
to noncrossover types among the progeny could be used as a measure
of the distances between two genes on a chromosome and that the use of
this measure made possible the construction of a linear map of the genes.
While Morgan had been vague about the relationship between association
of factors and their placement on the chromosome, Sturtevant proposed
a concrete way to quantify the distance between genes. Furthermore,
he claimed that if one knew AB (the distance between genes A and B) and
BC (the distance between B and C), then AC (the distance between A and
C) would equal their difference or their sum.
Gene mapping. From Hermann J. Muller, “The Mechanism of Crossing Over,” American Naturalist 50 (1916): 200.
By applying the same tests to strategically chosen combinations of
genes, he could make a map of the genes showing their linear order and