In Pursuit of the Gene

182 ¨ SEX CHROMOSOMES
somes, giving rise to half white-eyed sons and half short-winged sons. Astonishingly,
Morgan found that a small fraction of the sons had both white
eyes and short wings and another had neither.
In a brief paper submitted on October 19, 1910, he listed all the different
types of males and females that he’d seen in the second generation, including
the two new types, but he was still confused, writing that “the results
are difficult to explain fully.” 66 By the end of the year, he had found a way
to understand them. The new combination was possible, he wrote, “because
in the female of the hybrid (F1) a shifting of the gene for long and
that for short wing (both carried by the X chromosomes) takes place.” 67
Boveri had first suggested the idea on purely theoretical grounds in 1904,
writing, “the traits localized in a chromosome can go independently of
each other into one or the other daughter cell which would point to an exchange
of parts between homologous chromosomes.” 68 It was fitting that
Morgan, who was deeply wary of speculation, would be the one to provide
the experimental verification.
By July 1911, Morgan found further confirmation of the process of exchange
of parts of chromosomes, which he dubbed “crossing over,” in the
work of the Belgian cytologist Frans-Alfons Janssens. 69 Studying the formation
of sex cells under a microscope, Janssens observed that during the period
just prior to the nuclear division, after the matching chromosomes derived
from the mother and father formed pairs and began to move apart,
they appeared to be stuck together at certain points. Under the microscope
the strands appeared to form a crosslike figure, which Janssens called
a chiasma. 70
[To view this image, refer to
the print version of this title.]
Chiasma. From F. A. Frans Alfons Janssens, “La
Théorie de la Chiasmatypie: Nouvelle interpretation des
cinèses de maturation,” La Céllule 25 (1909): 400.