In Pursuit of the Gene

X-RAYS © 225
Suddenly Muller jumped up and called him over to look, his entire being
alight with excitement, as he held up a vial for Altenburg to examine. In
this culture there were no bar-eyed male progeny. Not only that, it was
also clear, as in the case of the beaded chromosome, that there was no
crossing over between the maternal X’s. 14 Instantly, Muller had surmised
that a novel mutant condition had arisen in the Bar-containing X of the
mother. C � , as he called the new mutation, simultaneously conferred two
new properties on the Bar X, acting both as a crossover suppressor and a recessive
lethal, resulting in the death of half of the grandsons. 15
Not only could he explain the absence of Bar grandsons in the anomalous
culture, but Muller had also seen how the new chromosome carrying
C � and Bar (soon to be known worldwide as the C � B chromosome) could
provide a simple new way to screen for spontaneously arising X-linked
lethals in male sperm; one would simply cross a male to a female carrying
one normal and one C � B chromosome. In the next generation, the F1,
those daughters containing the C � B chromosome, which could be identified
by the presence of the dominant Bar eye mutation, were individually
mated to their brothers, each in its own vial. Any F1 daughter that failed to
produce sons contained a new lethal mutation in her non-C � B chromosome,
the one inherited from her father.
P 1
F 1
F 2
C B
*
C B
C B
X
X
*
New lethal
mutation
* X
XC C B
X *
Dies
Dies
Method for detecting creation of new lethals using the C B chromosome.
Illustration by Tamara L. Clark.