THE ORIGIN OF VARIATION 



as should the entire field of elementary genetics, before going any further 

 in the study of evolution. 



THE GENE THEORY 



The name gene was applied to the hereditary unit by Johannsen with the 

 intention that it should be just a convenient term to designate the units 

 of heredity, which Mendel had referred to as "factors" or "elements." He 

 expressly stated that he proposed this term without intention of implying 

 any particular theory of the nature of the hereditary units. The name has 

 become closely associated, however, with the theory of T. H. Morgan 

 that the genes are corpuscular bodies in the chromosomes, arranged in 

 linear order like beads on a string, each gene separated from all of the 

 others and different from all of the others in substance, while all of the 

 genes of a particular chromosome are held together by an indifferent sub- 

 stance. This theory is based upon three types of evidence: the fact of 

 mutation; the fact that a linear order of genes in the chromosome can be 

 established by cross-over tests ( see Chapter 14 ) ; and the fact that, once 

 this order is established, it can be reshuffled by subsequent crossing over. 



This morphological concept of the gene dominated genetic thinking for 

 many years. Its boundaries were soon blurred by position effects ( Chapter 

 14); and more recently investigations of the biochemistry and physiology 

 of the gene have increased the difficulty of setting exact boundaries to it, 

 so that physiology rather than morphology dominates current thinking 

 about the gene. 



Gene Number and Gene Size. Attempts have been made to estimate 

 the number of pairs of genes for various organisms. Obviously, a direct 

 estimate of the number of genes is not available for any organism, both 

 because a complete study of all hereditary characters of a single organism 

 has never been made, and because a particular gene is identifiable by 

 genetic methods only when it is available in more than one form— in other 

 words, when it has mutated to form two or more alleles. Estimates for 

 the fruit fly, Drosophila melanogaster, genetically the best known of all 

 organisms, range from 5000 to 15,000. Belling has estimated about 2200 

 genes for Liliwn. And Curt Stern estimated not less than 5000 nor more 

 than 120,000 for man. Estimates of the size of the gene are based upon 

 the volume of the chromosomes (especially the euchromatic portions) 

 divided by the estimated number of genes. On this basis, Gowen and Gay 

 calculated the average size of the genes of Drosophila to be 1 X 10~^* 

 cubic centimeters. More recently. Pease and Baker, using the electron mi- 

 croscope, have observed in the salivary gland chromosomes of Drosophila 

 leaf-shaped bodies which they believe to be the genes ( Figure 74 ) . These 

 vary in size by a factor of about 3, but the average is about 1 X 10 ~"^' 

 cubic centimeters. As the salivary gland chromosomes are giant chromo- 

 somes to begin with, the mere fact that these bodies are ten times the size 

 estimated by Gowen and Gay cannot be regarded as conflicting with their 

 estimate. But neither is there any proof that these bodies actually are the 



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