CHAPTER XVII 

 CHROMOSOMES AND MENDELIAN HEREDITY 



Since the beginning of the present century the study of the role of 

 the chromosomes in heredity has been a major activity in biology. The 

 primary stimulus for such investigation came in the discovery that the 

 behavior of the chromosomes through the life cycle, particularly at 

 syngamy and meiosis, afforded an unmistakable clew to an explanation of 

 Mendel's laws of inheritance, which were rediscovered in 1900. Before 

 turning to the special evidence for the chromosome theory of heredity, 

 however, attention should be given to certain more general matters. 



Development and Heredity. — The problem of individual development 

 and that of racial heredity can never be wholly divorced. It should be 

 obvious that the course of ontogenetic development depends upon the 

 organization of the protoplast with which it begins, i.e., upon the type 

 of protoplasmic system concerned, and also upon the environmental 

 agencies which influence its action. The problem of development is, 

 therefore, to ascertain in what manner intrinsic and extrinsic factors, 

 together composing a single interacting system, operate to produce the 

 succession of changes which constitute ontogenesis: it is a problem per- 

 taining primarily to the individual. But any inquiry of this nature soon 

 leads beyond the individual life cycle to a consideration of the race and 

 hence to the problem of heredity. The protoplasmic organization upon 

 which the course of development so largely depends is itself an inheritance 

 from the past; indeed, the most fundamental fact which cytology has 

 contributed to the study of heredity is that the protoplasm of successive 

 generations is genetically continuous. These generations pass through 

 the same general series of ontogenetic stages and thus tend to develop 

 similar characters for the reason that the protoplasm with which each 

 ontogenetic cycle begins is of essentially the same constitution by virtue 

 of this continuity. Most characters are not literally "transmitted" 

 but are redeveloped in each generation. Generations may be regarded as 

 periodic developments of a persistent, though not unchangeable, proto- 

 plasmic system: the physical basis of heredity is protoplasm. 



It is not solely with resemblances that heredity is concerned. The 

 non-appearance of Mendelian characters in certain generations according 

 to definite rules, and hence the frequent unlikeness of parent and offspring, 

 are known to be just as dependent on the operation of intracellular 

 mechanisms as is the regular appearance of such characters in every 



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