CHAPTER XII 

 CYTOLOGY AND MENDELIAN HEREDITY 



For two thousand years and more, men have speculated on the causes 

 of Hkeness and difference between parent and offspring. The fact that 

 some parental characters are transmitted to the immediate offspring 

 while others are not, although the latter may reappear several generations 

 later, exemplifies the puzzles that could not have been solved until 

 recent times. Actuall}^ most of the characters exhibited by organisms 

 are not literally transmitted at all: they are developed anew in each 

 generation. The protoplasmic system which performs this development 

 is itself a direct inheritance from the previous generation; hence, what is 

 transmitted is a mass of protoplasm capable of developing the characters 

 under the appropriate environmental conditions. Since each generation 

 begins as a bit of protoplasm derived from the previous generation, 

 similarities between the two are expected, while differences must be due 

 either to the environmental conditions or to an actual constitutional 

 difference between the protoplasms with which the two generations begin 

 their development. Under uniform environmental conditions the 

 similarities and differences have been found to appear according to 

 definite rules. The inference from this is that there is in the protoplasm 

 an organized system of some kind that persists through successive 

 generations, yet undergoes minor orderly alterations affecting the char- 

 acters developed. The problem of modern cytogenetics is that of 

 describing this system, its transmission, its alterations, and its mode of 

 action in ontogeny. 



Before an adequate cytological theory of heredity could be devised, 

 certain prerequisites had to be furnished. (1) There was needed a more 

 precise formulation of the specific facts to be explained. This was 

 supplied by the famous nineteenth-century researches of Mendel, who 

 made mathematical analyses of the manner in which individual char- 

 acters were inherited after carefully controlled crosses. From these he 

 derived the laws that bear his name. (2) There was needed a more 

 detailed description of the organism's life cycle, especially through 

 the reproductive phases, in terms of visible structural units — cells, 

 nuclei, and chromosomes. This physical framework for a theory was 

 also largely furnished in discoveries made during the nineteenth centurj-: 

 the genetic continuity of protoplasm; the regular presence of the nucleus; 



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