CHAPTER XXVIII 

 THE PHYSICAL BASIS OF MENDELISM' 



ERNEST B. BABCOCK AND ROY E. CLAUSEN 



Recent investigations in heredity have focused attention upon the 

 chromoso me mechanism as the physical basis for the segregation and 

 recombination of the units of Mendelian inheritance. The importance 

 of cytological phenomena to students of genetics is admirably summed 

 up by E. B. Wilson in the brief statement that "heredity is a conse- 

 quence of the genetic continuity of cells by division, and the germ cells 

 form the vehicle of transmission from one generation to another." It 

 is appropriate, therefore, to introduce the subject of Mendelism with a 

 formal and brief treatment of the chromosome mechanism and its 

 mode of operation, on the one hand, in the building up of the body 

 from the single cell with which the individual begins its existence, and, 

 on the other hand, in the production of germ cells when the individual 

 reaches the reproductive period of its life cycle. It is the purpose of 

 this chapter merely to deal with the fundamental facts of cytology 

 which are necessary to an mderstanding of the connection between 

 cell behavior and Mendelian phenomena. Details unessential to such 

 an understanding, however well established cytologically, will not be 

 dealt with in this treatment to the end that the cardinal points may be 

 presented as simply and as clearly as possible. 



The chromosomes. — With few exceptions the number of chromo- 

 somes in the cells of any individual is constant and characteristic of 

 the species to which the individual belongs. Thus it is characteristic 

 of Drosophila ampelophila that the cells contain eight chromosomes. 

 In maize the cells contain twenty chromosomes, in wheat sixteen, and 

 in man forty-eight, and so on through the entire plant and animal 

 kingdoms. 



Not only is the number of chromosomes in a particular species 

 constant, but the chromosomes themselves possess a definite indi- 

 viduaUty. Man and tobacco have cells with the same number of 

 chromosomes. It is needless to point out that these chromosomes, 



' From E. B. Babcock and R. E. Clausen, Genetics in Relation to Agriculture 

 (copyright 1918). Used by special permission of the publishers, The McGraw- 

 Hill Book Company. 



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