240 



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 



Dwarf Parent 



Fig. 55. — Diagram illustrating behavior of chromosomes in Mendel's cross of 

 tall and dwarf peas. Large rectangular figures, nuclei of zygotes or mature in- 

 dividuals; large circles, gametes; small circles within zygotes and gametes, chromo- 

 somes; letters on chromosomes, determiners (T, tallness; D, dwarf ness). {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? In a 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 Figure 55. 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 x number, which 

 in this case is two. The diagram shows that the reduction separates 



