Genetic Analysis 261
tional flies were due to “chromosomal non-disjunction”, that is to an occasional
meiosis in an oocyte in which the two X-chromosomes moved to the same pole
rather than segregating to opposite poles (producing XX and no-X eggs, instead
of normal eggs with a single X). When such an oocyte was fertilized by a sperm
carrying a Y-chromosome a primary exceptional matroclinous daughter, carrying
XXY chromosomes was produced (and when a complimentary oocyte missing any
sex chromosome was fertilized by an X-chromosome-carrying sperm was produced,
a sterile primary exceptional patroclinous XO male was born — the “empty” Y-
chromosome presumably was needed only for fertility, if a male was born). Bridges
showed that hisgeneticanalysis of the chromosomal patterns of the flies was un-
equivocally confirmed incytologicalanalysis of the chromosome patterns. Bridges
analysis not only established the Chromosomal Theory of Mendelian Inheritance,
but also provided a firm foundation for genetic analysis as the major tool for the
phenomenological research of inheritance.
Bridges noted that if there was no preference of which possible pairs of the
three sex-chromosomes in XXY females was paired at meiosis — and consequently
was directed to segregate, the third unpaired chromosome always segregating at
random — a maximum frequency of 33.3% secondary non-disjunction may be
observed. The experimental results indicated that the two Xs pair (and segre-
gate) preferentially to each of the Xs and the single Y. His experimental tools did
not allow him to challenge this assumption. Almost thirty years later, in 1948
Kenneth Cooper showed that in stocks in which one of the two Xs is grossly rear-
ranged (and therefore the two Xs can hardly pair at meiotic prophase), as much as
90% secondary non-disjunction may be obtained. This could be avoided when a
one-armed Y-chromosome rather than the normal, two-armed Y chromosome was
used. He hypothesized that a triad, of a Y-chromosome, with each X-chromosome
paired to (and segregating from) another arm of the two armed Y-chromosome
was produced, rather than two chromosomes pairing and the third moving at ran-
dom. Such triads may give up to 100% secondary non-disjunction since both Xs
are directed to segregate to the same pole,away from the Y-chromosome [Cooper,
1948], (see also Falk [1955]). Of course, no indications of the physical and/or
chemical forces that cause specific pairing and segregation of the chromosomes
were indicated. These were open to wide and often rather wild speculations.
Once the chromosomal theory of heredity was established, cytogenetics became
an integral part of genetic analysis. Already in 1914 Muller discovered a fourth
small linkage group inDrosophila melanogaster, as expected from the cytological
observations of the flies, which own, besides the sex-chromosomes and two major
autosomal chromosome pairs, also a pair of minute chromosomes [Muller, 1914].
Bridges suggested that the dominant sex-linked mutation that notched the margin
of the wings in heterozygous females and that was lethal in males was actually
a physical deficiency of a chromosome section [Bridges, 1917], although at the
time it was below the resolution power of cytological techniques. In stocks that
normally carried markers that segregate independently, dependent segregation of
these markers in genetic experiments was indication for presumptive chromosomal