Evolution of Knowledge Encapsulated in Scientific Definitions

"When I use a word," Humpty Dumpty said, in rather a scornful tone, "it means just what I choose it to mean--neither more nor less."

"The question is," said Alice, "whether you can make words mean so many different things."

"The question is," said Humpty Dumpty, "which is to be master--that's all."

--Lewis Carroll, Through the Looking Glass, 1871

A number of writers have observed that every Age of Man deludes itself in thinking it is the most enlightened in the history of the world. Although this no doubt applies to the biological scientific community in the first decade of the 21st century, confusion still abounds with respect to the definition of important scientific terms. An example: although one of the greatest scientific achievements of all time is said to be the much-heralded sequencing of the human genome, there is still no general agreement on the definition of a gene. This essay examines the changing history of two definitions of special importance in the biological sciences.

It is reasonable to assume that as the essence of a scientific phenomenon or entity becomes elucidated over the course of many decades, its definition changes accordingly. Otherwise, we would hardly need new editions of dictionaries. I have a number of dictionaries in my office, but tend to use a "standard college dictionary" (Funk & Wagnalls 1963) for routine writing purposes. Recently, I looked up the definition of gene in this source and was surprised to read: "One of the complex protein molecules associated with the chromosomes of reproductive cells and acting, as a unit or in various biochemically determined combinations, in the transmission of specific hereditary characters from parents to offspring." This dictionary was published 19 years after the research of Avery, et al., demonstrated that the genetic material is DNA (McCarty 1985), and 10 years after the famous Watson-Crick paper on DNA structure appeared.

The faulty definition of a gene was particularly striking to me because for more than a decade I have been concerned with improving the definition of photosynthesis. A satisfactory definition of this process is not given in most dictionaries, even in important reference works such as the Oxford English Dictionary (OED 1989). I concluded that it would be instructive to trace, as examples, the changing definitions of the terms gene and photosynthesis.

Gene

The word gene was proposed by W.Johannsen (1909) in a German text of 25 lectures. In a paper written in English two years later (1911), Johannsen described the use of the word gene as follows: "Therefore I have proposed the terms 'gene' and 'genotype' and some further terms, as 'phenotype' and 'biotype' to be used in the science of genetics. The 'gene' is nothing but a very applicable little word, easily combined with others, and hence it may be useful as an expression for the 'unit factors', 'elements' or 'allelomorphs' in the gametes, demonstrated by modern Mendelian researches" (p. 132).

Some later definitions follow:

1945: "An entity concerned with the transmission and development or determination of hereditary characters; an element of the germ plasm, regarded as a small part of a chromosome; a factor or determiner" (Webster's New International).

1971: "The term 'gene' no longer stands simply for a discrete structural unit of heredity of definite and invariable length. It is now thought of as an operational entity whose properties depend upon the mode of measurement: mutational capability; recombinational performance; or functional activity" (Encyclopedia Britannica).

1989: "Each of the units of heredity which (except for polygenes) may be regarded as the controlling agents in the expression of single phenotypic characters and are usually segments of a chromosome at fixed positions relative to each other; they were originally defined as ultimate units of mutation and recombination, but are now best regarded as sequences of nucleotides within nucleic acid molecules each of which determines the primary structure of some protein or polypeptide molecule.The term...