In Pursuit of the Gene

These strange X’s, Janssens speculated, might be the result of an exchange
between chromosomes, giving rise to hybrid, partly maternal, partly
paternal chromosomes. Morgan saw that he could connect the physical
evidence of crossing over presented by Janssens with the results of his
breeding data. Factors on matching pairs of chromosomes could be exchanged,
Morgan suggested, and the chiasmata that Janssens had seen under
the microscope were the physical manifestation of crossovers between
these matched pairs. The result of such a crossover was the creation of two
new hybrid chromosomes.
Crossover between (W) and
(S). Illustration by Tamara L. Clark.
SEX CHROMOSOMES © 183
Furthermore, Morgan hypothesized, the closer to each other two
genes were located in the chromosome, the less likely it was that a crossover
would take place between them. “Instead of random segregation in
Mendel’s sense,” Morgan asserted, “we find associations of factors that are located
near together in the chromosomes.” 71 Conversely, genes farther apart
would have more crossing over between them. It was thus that Morgan,
one of Mendel’s most vociferous doubters, supplied the first significant refinement
of Mendelism in forty-five years.
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W S