Philosophy of Biology

Genetic Analysis 265

Carl Correns was probably the first to take notice of the phenomenon of the
deviation from the rule of independent segregation of different traits, and to put
forward a physiological explanation of differential lethality of gametes [Correns,
1902). But it were Bateson and Punnett who elaborated on the phenomenon
and proposed a theory of “coupling” and “repulsion” — terms borrowed from
electro-magnetic theories — of Mendelian factors along Bateson’s 1891 “Theory
of Repetition of Parts.” Some gametes, but not others, were “plainly a conse-
quence of some geometrically ordered series of divisions” [Bateson and Punnett,
1911]. Bateson, the most keen proponent of the particulate mechanistic theory of
inheritance — see his opposition to Pearson’s biometrical notions — adopted a
non-mechanistic approach when confronted with deviations from independent seg-
regation of these factors: As he did with the metameric phenomena he had been
dealing with earlier, he interpreted the deviations from Mendel’s rule in terms of
field-theories, inspired by the electro-magnetic field-theories of Faraday and oth-
ers that heralded the demise of the mechanistic paradigm and the resurgence of
the organicistic conception (Coleman [1970], see Gilbertet al. [1996]) for the
establishment of the biological field theory.
Contrary to Bateson, Punnett and their colleagues’ complete oblivion to cyto-
logical observations, Morgan found that “[t]he discovery that there occurs in the
formation of the germ-cells a process that supplies the machinery by means of
which segregation might take place has aroused... interest in the application of
the observations of cytology to the conclusions in regard to Mendelian segregation”
[Morgan, 1910a, 465]. For Morgan the Chromosomal Theory of Inheritance pro-
vided the resolution to the phenomenon of deviation from independent segregation
of trait, or linkage [Morgan, 1911].

I venture to suggest a comparatively simple explanation based on re-

sults of inheritance of eye color, body color, wing mutations and the

sex factor for femaleness inDrosophila. If the materials that represent

these factors are contained in the chromosomes, and if those factors

that “couple” be near together in a linear series, then when the parental

pairs (in the heterozygote) conjugate like regions will stand opposed.

... [W]hen the chromosomes separate (split)... the original material
will, for short distances, be more likely to fall on the same side of the
split, while remoter regions will be as likely to fall on the same side as
the last, as on the opposite side....
[W]e find coupling in certain characters, and little or no evidence at all
of coupling in other characters; the difference depending on thelinear
distanceapart of the chromosomal materials that represent the factors.
... The results are a simple mechanical result of the location of the
materials in the chromosomes,... and the proportions that result are
not so much the expression of a numerical system as of therelative
locationof the factors in the chromosomes. [Morgan, 1911]

For the chromosomal theory of inheritance to be the explanation of linkage,