CHAPTER VII 
THE NATURE AND EXPRESSION OF MENDELIAN FACTORS 
In previous chapters the formal relations which exist in the trans- 
mission of factors from parent to offspring have been discussed. It has 
been shown that these relations may be ascribed to the locus positions 
which factors occupy in the chromosomes. This single assumption 
taken together with the known behavior of the chromosome mechanism 
in its cycles explains very simply the two great categories of inheritance 
with respect to distribution of factors, namely independent segregation 
and linkage. Obviously, however, these are merely formal considerations, 
it is of considerable importance to know something about the factors 
themselves and the physiological interactions which they display with 
one another in the development of characters in the individual. It is 
to this problem that this and following chapters are addressed. 
It is true that as yet we know next to nothing about the factors 
themselves with respect to their physical and chemical constitution, we 
know them merely by their actions. We regard them as the loci ar- 
ranged in a linear series in the chromosome, we know they have certain 
characteristic effects in development and by these effects we recognize 
them. It is important to note that our knowledge of their behavior 
even is based on factor differences, not on a study of the factors them- 
selves. Thus we know that a certain locus in the germinal substance 
in Drosophila is concerned with the production of red eye color because 
when it is changed in a particular fashion, the eye color developed is 
no longer red, but white. We have no means of knowing how profound 
the relation of this factor to the other factors in the system is, nor can 
we judge as to the nature of the change in the locus by which the course 
of development was shifted from red to white in the production of eye 
color in Drosophila. Nevertheless a few things at least are known con- 
cerning the effects of factors in development and even in this vague field 
more and more facts are being discovered all the time. 
Factors are the genetic representatives of certain characters. Thus if 
a fly has a genetic constitution containing, among other factors for eye 
color, the factor w, then it will develop white eyes. In this particular 
case the eye color is practically the only character affected. Similarly 
in corn, if a mutation occurs in one of the basic aleurone color factors, 
for example, a change in the chromogen factor C to c then that corn thus 
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