388 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
something is called by various names in the literature of genetics, the 
commonest one being determiner. In our illustration, therefore, two 
of the four chromosomes carry the determiner for tallness. At this 
point two questions may be asked. 
1. Why do just two of the four chromosomes carry the determiner 
for tallness rather than all of them or only one of them? Just here 
it would be difficult to explain why no more than two of the four 
chromosomes are represented as carrying the same determiner. This 
will be explained later. It is easy to answer, however, why the deter- 
miner is being carried by more than one chromosome. When gametes 
are formed the chromosome number is reduced one-half. Since every 
gamete from a pure tall plant carries the determiner for tallness there 
: B8.x5 
Tall Parent 
Dwarf Parent 
Fic. 67.—Diagram illustrating behavior of chromosomes in Mendel’s cross 
of tall and dwarf peas. Large rectangular figures, nuclei of zygotes or mature 
individuals; large circles, gametes; small circles within zygotes and gametes, 
chromosomes; letters on chromosomes, determiners (7, tallness; D, dwarfness). 
(From Coulter and Coulter.) 
must have been at least two chromosomes carrying the determiner 
before the gametes were formed. 
2. Do these two chromosomes carry any other determiner than 
that for tallness? Ina tentative way this question may be answered 
in the affirmative, but a fuller discussion of the situation must be 
deferred. There is much experimental evidence that indicates that 
more than one determiner is carried on a single chromosome. In some 
cases also there are more Mendelian determiners than there are 
chromosomes. 
The situation is represented in Fig. 67. This shows a somatic 
cell with the diploid or 2x number of chromosomes. In the formation 
of gametes this number is reduced to the haploid, or « number, which 
in this case is two. The diagram shows that the reduction separates 
