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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