262 Raphael Falk
reciprocal translocations. Alsovice versa, cytologically detectable major chro-
mosomal aberrations — primarily reciprocal translocations — were confirmed by
genetic analysis of deviation from the independent segregation of the markers in
Drosophila and maize, especially after it was shown that X-rays were efficient
means to induce aberrations (see section 4).
Aberrations became a major tool for the analysis of chromosomal mechanics.
Thus, for example, it was shown that in triploid Drosophila flies — having three,
instead of two sets of each chromosome — although all three chromosomes may
be paired to each other in meiosis, only two of the three homologues pair ateach
sitealong the chromosome, and the pairing site most proximal to the centromere
was the most significant in indicating the chromosomes that segregate to oppo-
site poles [Anderson, 1925; Bridges and Anderson, 1925], supporting cytological
conclusions that the centromere was the site that leads chromosome segregation.
The availability of large chromosomal inversions or translocations that were shown
cytologically to interfere with chromosome-pairing, and that were detected genet-
ically to induce non-disjunction and recombination-suppression (see section 4),
further established the need for chromosome pairing in meiosis in determining
chromosome segregation [Dobzhansky, 1929; Dobzhansky, 1931].
Although some species like corn (Zea mays) had both relatively large chromo-
somes and numerous genes that were located on these chromosomes, most exper-
imental species had either large chromosomes but few genes that were located on
them (e.g.,Lilium sp.), or abundant gene markers but chromosomes too small
to discern any but the most conspicuous structural changes. The latter was the
case withDrosophila melanogaster, the major research object of Morgan and his
associates. This changed dramatically when it was shown that the ‘spereme’ —
“the discoid structure of the chromosomes” of the Malppighian tubules (and the
salivary glands) in Dipteran larvae [Kostoff, 1930], was in reality a discontinu-
ous structure, the number of components of which corresponded to the haploid
number of chromosomes [Heitz and Bauer, 1933; Painter, 1934], (see also Falk
[2003, 106–107]). These turned out to be highly uncoiled, homologously paired
polytenic chromosomes due to repeated endomitoses (without separation of the
replicated polytenes). Thus providing extremely powerful cytogenetic magnifica-
tion and resolution of the fly’s chromosomes, since identity in the coiling outline
of all replicated (and paired) strands gave a typical pattern of “bands” and “inter-
bands” across the polytenous structures. This allowed the construction of detailed
linear cytological maps of the chromosomes ofDrosophila melanogaster[Bridges,
1935; 1938; Bridges and Bridges, 1939]. Maintaining the pairing of homologous
“bands” in hybrids for chromosomal rearrangements forced typical chromosomal
configurations, such as loops (for inversions), Ω-like structures (for deletions), or
crosses (for translocations), etc., which made heterozygotes of chromosomal aber-
rations effective tools for superposing of the genetic and cytological maps. Further
striking support for the chromosomal theory of heredity was the assignment of
genes to specific “bands” of the polytenic chromosome map. For example, fol-
lowing a series of white-eyed stocks of Drosophila, which turned out to be small