312 INTRODUCTION TO CYTOLOGY 



possible. As pointed out in an earlier chapter (p. 141) the chromonema 

 often exhibits a series of chromomeres arranged in a characteristic 

 pattern, and it is expected by many observers that these bodies may be 

 shown sooner or later to have some definite and significant relation to the 

 genetic units known as genes, as has long been conjectured. Whether 

 or not improved optical methods will bring visual confirmation of the view 

 that the gene is a discrete material unit, it will still remain true that it is 

 a distinct constitutional feature which is definitely localized in the nucleus 

 and which produces certain specific effects during the life of the organism. 

 Hence the geneticist can treat it as a unit, whatever its ultimate physico- 

 chemical nature may be. He may thus make effective use of it, no matter 

 how he may regard certain speculative hypotheses^^ suggesting that genes 

 are ultimate living particles which have somehow aggregated and differ- 

 entiated to form chromosomes, nuclei, and organisms. 



The expression "factorial theory" usually has reference to the con- 

 ception of Mendelian factors or genes, but it should be obvious that any 

 comprehensive interpretation of organic phenomena in terms of contribu- 

 tory causes must include the action of other factors as well. It is to be 

 strongly emphasized that the system in which the phenomena of heredity 

 and development occur is the organism-environment. There is probably 

 no character which can be attributed to the organism or to the environ- 

 ment alone; these two components of the system cannot be separated in 

 any real sense. What an organism inherits is not a set of characters but a 

 protoplasmic organization giving it the capacity to react in certain ways 

 under certain conditions, the developed characters being the result of the 

 action of the entire system of which the organism and environment are 

 integral parts. The relative differential effects of genetic and non- 

 genetic factors may be expected to vary under appropriately varying 

 circumstances.^* 



Summary. — According to the chromosome theory of heredity, 

 each chromosome carries a linear series of genes, which are units having 

 specific effects upon the course of development and hence upon the 

 morphological and physiological characters of the organism. Characters 

 dependent on genes in the same chromosome tend to be inherited together 

 as a linked group. A gene may be constant in its composition and effects, 

 or it may occasionally mutate and therefore exist in two or more allelo- 

 morphic states. In the latter case its role may be studied in crosses. 



Two sets of chromosomes and hence two lots of genes are brought into 

 association in syngamy. When the resulting diploid organism forms 

 gametes (in most animals) or spores followed later by gametes (in most 

 plants), the homologous chromosomes and therefore the corresponding 



23 See Alexander and Bridges (1928) and Darlington (1932a, Chap. XVI). 

 " For discussions of this point, see Emerson (1924), Jennings (1924, 1930), and 

 Sharp (1925). For a full account of the modern gene theory, see Morgan (1928). 



