266 Raphael Falk
it was necessary to spell out why linkage was not absolute, or, why all factors
linked to a given chromosome should not segregate as a unit. Since “coupling”
and “repulsion” were not absolute, Morgan had to add the notion of crossing over,
which he borrowed from Janssens’ chiasmatype theory, based on observations of
meiosis at spermatogenesis in the Batarchine salamander [Janssens, 1909]. During
meiosis homologous chromosomes (each had already replicated to form two chro-
matids) pair, and are increasingly coiled and twisted one about the other up to the
pachytene stage. At the diplotene stage the tension appears to be relieved and the
paired chromosomes start to segregate, except at some sites, at which sister chro-
matids appear to cross over. Janssens hypothesized that the mechanical twisting
leads to breaks in the paired chromosomes, allowing some relaxation of the tension
by local untwisting, which is followed by repair of the broken “sticky” ends, at least
some of such repair occurs between the paired-chromosomes. This mechanism of
exchange provided for the partial linkage that Morgan had to explain.
We may make a general statement or hypothesis that covers cases like
these, and in fact all cases where linkage occurs: viz. that when factors
lie in different chromosomes they freely assort and give the Mendelian
expectation; but when factors lie in the same chromosome, they may
be said to be linked and they give departures from the Mendelian
ratios. The extent to which they depart from expectation will vary
with different factors. I have suggested that the departures may be
interpreted as the distance between the factors in question. [Morgan,
1913b, 92-93]Thus, when Sturtevant harnessed genetic analytic experimentation to construct
linkage-maps, he had in mind the mapping of genes along chromosomes [Sturte-
vant, 1913]. But these were actually virtual maps, the physical meaning of which
was not at all generally accepted. Richard Goldschmidt pointed out that describ-
ing linkage data in terms of a linear map is merely the application of a standard
procedure of analytic geometry. He doubted the inspiration based on the cyto-
logical observations of Janssens and suggested instead a physiological mechanism
in which an “immune-like” attraction between chromosomes may occur at repli-
cation, the strength of linkage expressing the intensity of the reaction rather than
the physical distances between sites [Goldschmidt, 1917]. Muller, however, suc-
ceeded in examining recombination in specially constructed multiply-linked genetic
marked Drosophila stocks. He demonstrated that “factors behave as though they
are joined in a chain; when interchange takes place, the factors stick together in
sections according to their place in line and are not interchanged singly”. Accord-
ingly linkage is a phenomenon of the topology of the genes rather than a property
of individual genes [Muller, 1916, 366]. Regarding William Castle’s doubts “[t]hat
the arrangement of the genes within a linkage system is strictly linear seems for a
variety of reasons doubtful”, for example, it was doubtful “whether an elaborate
organic molecule ever has a simple string-like form” [Castle, 1919b, 26], Muller
pointed out that the genetic analysis of linkage was a phenomenological study.