MEN DELI SM AND MUTATION 



between the distribution of a given homologous pair of chromosomes and 

 thai of a single allelomorphic pair of Mendelian characters. 



This is exactly the condition which would result if two material 

 units, each representing one of the characters of an allelomorphic pair, 

 were located in two homologous chromosomes that pair and separate at 

 reduction. The chromosomes afford precisely the type of mechanism 

 required to account for the distribution of characters if the latter are 

 associated with a definite material basis. It is this parallelism between 

 the behavior of the chromosomes in reduction and that of Mendelian 

 factors in segregation, first emphasized by Boveri and by Button, which 

 has led geneticists generally to the view that the characters are actually 



FIG. 134. Diagram showing the 16 genotypic constitutions which may be present in the 

 gametes of an organism with only 4 pairs of factors. (After Wilson, 1913.) 



represented in the chromosomes by material factors, or genes, which in 

 some unknown manner control the development of the characters in 

 the body. 



The earlier view that each character is thus represented by a single 

 material unit or determiner has now given way to the more fully devel- 

 oped Factorial Hypothesis, according to which, on the one hand, a 

 character may be due to the cooperative action of two or more factors 

 ("duplicate" or "cumulative" factors); and, on the other hand, a single 

 factor may have "manifold effects," influencing the development of several 

 characters. The factors, or genes, are thought by some to constitute a 

 complex reaction system, interactions between genes having a marked 

 effect upon their activity in producing characters. 1 "The factorial 



1 A simple and brief explanation of the effects of cumulative factors is given by 

 Coulter and Coulter (1918). 



