CHAPTER IV 

 THE PHYSICAL BASIS OF MENDELISM 



Recent investigations in heredity have focused attention upon the 

 chromosome mechanism as the physical basis for the segregation and re- 

 combination 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 consequence of 

 the genetic continuity of cells by division, and the germ cells form the ve- 

 hicle 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 under- 

 standing of the connection between cell behavior and Mendelian phe- 

 nomena. 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 chromosomes 

 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 con- 

 stant, but the chromosomes themselves possess a definite individuality. 

 Man and tobacco have cells with the same number of chromosomes. It 

 is needless to point out that these chromosomes, however, are quali- 

 tatively very different Similarly within the species the chromosomes are 

 not all alike; on the contrary, especially in certain forms, they exhibit 

 very marked differences in size and shape. This is peculiarly well illus- 

 trated in Drosophila as shown in Fig. 27. Here it is possible to recog- 

 nize in the female two large pairs of curved chromosomes very similar 

 in size and shape. There is also a very small pair of chromosomes, and 



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