274 Raphael Falk
or chromosomal-event. If, however, the notions expressed in the “target theory”
paper (known sometimes as the TZD experiment, in covert analogy to the EPR
experiment by Einstein Podolsky and Rosen in physics) had the claimed impact
on the physicists community to turn to biology for laws of nature not detected in
physics research [Stent, 1968], then its bearing on the upcoming molecular biology
age is profound indeed.
Although the research program to deduce the physico-chemical nature of genes
from the genetic analysis was extended to include the study of chemically induced
mutations, it turned out that this too did not resolve the nature of the genetic
material (see section 5.1 below).
As Muller anticipated, many problems, like that of the nature of induce mu-
tations, disappeared with the molecular genetic analysis. Once the chromosomes
were recognized as continuous strand of nucleic-acids, the extended dispute of
whether induced mutations were genuine “point mutations” became immaterial:
In such a molecule there is no difference between, say, a deletion of one nucleotide
or its conversion into another nucleotide.
Morgan’s emphasis in his Nobel Lecture that for all practical purposes it did
not make any difference what the physico-chemical nature of the genes were, may
be taken as a subtle rebuke of Muller’s research program:
Now that we locate [the genes] in the chromosomes are we justified in
regarding them as material units; as chemical bodies of a higher order
than molecules? Frankly, these are questions with which the working
geneticist has not much concerned himself, except now and then to
speculate as to the nature of postulated elements.... at the level
at which the genetic experiments lie, it does not make the slightest
difference whether the gene is a hypothetical unit, or whether the gene
is a material particle. [Morgan, 1935]Nevertheless, in spite of the failure to determine by genetic analysis the nature
of the gene, Muller’s focus on this problem widely opened the path of genetic
analysis as a method for resolving the role of inheritance in organisms’ function
and development, as well as in evolution (see, e.g., Falk [1997]), in an effort to
understand “the gene as the basis of life” [Muller, 1929; 1947]. In that sense
he laid the foundations for the molecular age of genetic analysis (see Lederberg
[1991]). Yet, the problem that Muller put up, as cited at the beginning of section
5, whether individual genes are separate bodies or only convenient mental isolates,
prevails and obtains new significance at the age of genomics [Muller, 1929; 1947],
(see Beurtonet al.[2000]; Burian [2004]; Falk [2004]; Moss [2003]).
Beadle and Tatum’s notion of a one-to-one relationship between genes and en-
zymes, the “one gene — one enzyme” slogan [Beadle and Tatum, 1941a, 1941b],
was one direct consequence of the notion of genes as distinct entities along the
chromosome. So was the notion of population genetics, the mathematics models
of which, starting with the Hardy–Weinberg’s rule of 1908, related to genes as the
indivisible atoms of heredity (see section 8).