CHAPTER XXVIII 
THE PHYSICAL BASIS OF MENDELISM! 
ERNEST B. BABCOCK AND ROY E, CLAUSEN 
Recent investigations in heredity have focused attention upon the 
chromosome 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 understanding 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- 
viduality. Man and tobacco have cells with the same number of 
chromosomes. It is needless to point out that these chromosomes, 
t 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. 
AQT 
