In Pursuit of the Gene

the relative distances between them. In his History of Genetics Sturtevant described
this banner day with his characteristic dry understatement:
I went home and spent most of the night (to the neglect of my undergraduate
homework) in producing the first chromosome map, which included
the sex-linked genes y, v, m, and r, in the order and approximately
the relative spacing that they still appear on the standard maps. 23
In fact, Sturtevant’s proposition that AB + BC = AC (when B lay between
A and C) corresponded only approximately to the actual data based
on the numbers of crossover and noncrossover types, and AC was typically
somewhat less than AB + BC. In order to account for the discrepancy,
Sturtevant suggested that a chromosome occasionally underwent two
crossovers simultaneously, the first between genes A and B, and the second
one between B and C.
[To view this image, refer to
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THE FLY ROOM © 193
Double crossover. From Hermann J. Muller, “The Mechanism of Crossing Over,” American Naturalist 50 (1916): 202.
In the cases of a double crossover, as Sturtevant called it, A remained linked
to C (though no longer to B). Since each double crossover actually represented
two single crossovers, the number of crossovers between two genes
A and C (and consequently the distance between them) would tend to be
underestimated.
Muller, who was in the midst of his second fellowship year in physiology
at Cornell Medical school, first learned about Sturtevant’s idea at one
of the Friday night get-togethers at Morgan’s house where Morgan prepared
fried squid Neapolitan style and a small group of students read recent
papers (of Morgan’s choosing) out loud. Though it was Morgan’s policy to
restrict the discussion those evenings to topics other than genetics, he
made an exception for Sturtevant’s gene-mapping technique. Muller, who