260 Raphael Falk
Confirmation of the independent segregation of chromosome pairs was obtained
in meioses ofB. magnamales that possessed, besides the single X-chromosome,
another pair of chromosomes that could be individuated by the difference in their
size: In 146 cells the smaller element of the unequal pair segregated to the same
pole as the X-chromosome in meiotic anaphase, whereas in 154 the larger element
segregated with the X-chromosome [Carothers, 1913].
Notwithstanding, all these studies did not prove the causal role of chromo-
somes in heredity and development. Chromosomes could be just another cell-
morphology trait that segregates as Mendelian traits do. However, the observa-
tion of a white-eyed fruit fly, aDrosophila melanogastervariant whose progeny
demonstrated in extensive hybridization experiments the “criss-cross” pattern of
heredity (white eyed females mated to wild-type, red eyed males produced wild-
type, red eyed daughters and white sons) as does the cytologically observed X-
chromosome [Morgan, 1910b], initiated the intensive analysis of the mechanisms
of inheritance of the Morgan School. Their working-hypothesis was that chromo-
somes play a causal function in the transmission and development of traits (except
for the Y-chromosome that presumably was “empty” or inert — see section 6).
Several other mutant stocks besides thewhiteone were established (mutants are
designated by italicized letters,w), each of which carried a mutant that was not
allelic towhite.^2 Yet they too were inherited in the “criss-cross” pattern, like
the cytologically observed X-chromosome. This strongly supported the hypothesis
that the cytologically observed maneuvers of chromosomes are the physical cause
for the segregation pattern of these genes, and that the mutant traits of these
stocks were due to mutations in different loci on the X-chromosome — loci of
“sex-linked” genes. Other mutants of Drosophila could be grouped into two major
non-sex-linked or “autosomal” linkage groups, presumably related respectively to
each of the two pairs of larger chromosomes of the fly. Put differently, these find-
ings pointed to the Chromosomal Theory of Inheritance: Chromosomes were the
carriers of the genes, whatever the nature of these genes might be.
Calvin B. Bridges’ paper that introduced the newly established journalGenet-
ics, providing the experimental evidence for the chromosomal theory of inheritance,
may be considered as the flagship of the methodology of genetic analyses [Bridges,
1916]. Rare exceptions to the “criss cross” inheritance of the characteristic sex-
linked “markers” were observed: Instead of the expected “wild-type” daughters
and vermillion sons from a cross ofvermilion(v) females to wild-type males, an
occasional vermillion daughter (or a wild-type son) was obtained. Crossing the pri-
mary exceptional vermillion daughter to regular wild-type males produced 3-4%
secondary exceptional progeny, rather than the rare primary exceptions (primary
exceptional males were sterile). Several females and males of the progeny contin-
ued in their turn to regularly produce secondary exceptional progeny (compared
to the “criss-cross” expectations). Bridges hypothesized that the primary excep-
(^2) As shown by “complementation tests”, i.e., the compound heterozygotesa/bwere pheno-
typically non-mutant or “wild type”. Different mutants that do not complement are allelic and
designateda 1 /a 2 — see also section 5