Genetic Analysis 267Whether or not we regard the factors as lying in an actual material
thread, it must on the basis of these findings be admitted that the
forces holding them linked together — be they physical, ‘dynamic’ or
transcendental — are of such nature that each factor is directly bound,
in segregation, with only two others — so that the whole group, dy-
namically considered, is a chain.... no implication as to the physical
arrangement of the genes is intended when the terms ‘linear series,’
‘distance,’ etc., are used. [Muller, 1920, 101]
Muller’s statement notwithstanding, the correlation of the virtual linkage map
and the physical chromosomal map was further supported by studies like that of
Mohr [1924] on theNotch-“deficiency”. As noted (in section 3), Bridges [1917]
suggested that the dominant sex-linked, recessive lethal mutationNotchwas ac-
tually a physical deficiency of a chromosome section. Notched females mated to
white-eye males or to facet-eye males (wandfaare two recessive mutations at
different but closely linked genes) produced notched-white and notched-facet fe-
males respectively. Mohr showed that the frequency of crossing over between genes
lying to the left and to the right of the “deficiency” was significantly reduced in its
presence. Once polytenic chromosomes of Drosophila (and other dipterans) were
mapped, it was straight forward to correlate the genetically observed changes of
the virtual linkage maps with cytological events. Thus, the genetic marker Notch
turned out to be a deficiency of band 3C7. And the well knownC-factors, or
suppressors of recombination of genetic analysis (like in theClBstockofMuller)
turned out to be inversions of at the cytological level (see Lindsley and Grell
[1968]). Most intra-inversion crossing-over events in hybrids of an inverted and a
non-inverted chromosome produce upon recombination aneuploid, non-viable de-
ficient or duplicated chromosomal products, and are consequently efficient genetic
“suppressors” of recombination.
According to Janssens’ mechanistic conception that was adopted by Morgan,
linkage was primarily a function of the probability of breakage of the twisted chro-
mosomes; the further apart genes were positioned on the chromosome the higher
the probability of a random breakage and recombination event, and the looser the
linkage (when it turned out that the frequencies of genetic recombination were
not homogeneously distributed over all cytological unit length, this was modified,
claiming that the structure of the chromosomes was not uniform, so that there
were sections that broke more readily than others). Bridges [1916] analysis of
non-disjunction indicated that exchange involved only two of the four strands at
every recombination site. This was confirmed by L. V. Morgan who showed that in
a Drosophila stock in which the two X-chromosomes were physically attached (and
accordingly all their daughters were obligatory X-chromosome non-disjunctional,
i.e., carried two of the four products — half a tetrad — from thesamemeiotic
event), some daughters were homozygous for genetic markers for which the mother
was heterozygous. This could be obtained only if cross-over occurred after chromo-
some replication between two of the four paired chromatids, when a non-crossover
and a cross-over chromatid happened to disjoin. Furthermore, since according to