Biology Reference
In-Depth Information
nature of the causal process that connects the gene and
the characters”
50
(Morgan, 1926, pp. 25-27).
However, Morgan very quickly realised that this determinism
was not compatible with experimental reality. Several phenomena
blurred the correspondence and causal relationship between the
gene and the characteristic. In pleiotropy, a gene affects several dif-
ferent phenotypic characteristics, while in polygeny, it is the
reverse, with several genes affecting a single characteristic. There is
also the conditional expression of a gene. A characteristic depends
on a gene, but it is only expressed under certain environmental con-
ditions, e.g. at a given temperature. More recently variable expres-
sivity has been shown. A gene corresponds to several phenotypic
characteristics which are expressed with a certain frequency in a
population of organisms. Because of these phenomena, the simple
correspondence between genes and characteristics that Morgan
talked of in
The Theory of the Gene
is very difficult, if not impos-
sible, to establish. However, this is one of the pillars of genetics. If
it is not possible to establish the map of causal relationships
between genes and characteristics, genetics is invalid as an explana-
tory theory. For it to remain significant, the relationship between
the gene and the phenotypic characteristic cannot be reduced to a
simple statistical correlation seen empirically. This led Morgan to
write another topic,
Embryology and Genetics
(1934), in which he
tackled this question and re-evaluated the importance of embryonic
development in heredity. Here is what he said in 1934, less than
10 years after publishing
The Theory of the Gene
:
“In the early days of genetics, i.e., at the beginning of the century,
'unit characters' were supposed to furnish the basis for genetic
work, and by inference each gene was supposed to produce a spe-
cific effect in only one character at a time. This premature infer-
ence was very soon found to be erroneous when the manifold effects
of each genic change came to be known. It is true that in most
50
Original text not in bold.
