Chromatin as "Hereditary Substance" 359 



each chromosome of any series, has a corresponding one in any 

 other series, and if these have such an identity and freedom 

 to combine and separate as the discussion had assumed, then 

 if a given chromosome of the father be designated by A and its 

 mate in the mother by a, in synapsis there would arise Aa, which 

 on reduction division preparatory to ripening of the eggs and 

 sperm would produce two kinds of eggs and two kinds of sperm 

 relative to this set of chromosomes, namely male A and female 

 /] ; and male a and female a. These if equal in number and 

 equally free in their movements would give in fertilization : 

 Male A -f- Female A = A A 

 " A + " a = Aa 



a - A = aA 



a -f- a = aa 



Or since Aa and a.l are alike the expression becomes AA -f~ 

 2Aa -f- aa as the distribution, or again A -{- Aa -\- a as the 

 sum of possibilities of each chromosome pair. "Thus," Sutton 

 says, "the phenomena of germ-cell division and of heredity are 

 seen to have the same essential features, viz., purity of units 

 (chromosomes, characters) and the independent transmission of 

 the same." 24 



We must now return to the truly remarkable discoveries 

 made by Morgan and his students and collaborators on mu- 

 tations in Drosophila and on the behavior of the mutant 

 attributes in heredity. These have consisted in showing that 

 such a relation between attributes and chromosomes as that 

 assumed in this relatively simple scheme worked out by 

 Sutton may be carried out in much detail both as to attri- 

 butes and chromosomes. An especially ingenious and fas- 

 cinating aspect of the theory as it has been elaborated large- 

 ly under Morgan's leadership, is that which shows the 

 possibility that different parts of one and the same chromo- 

 some may correspond to several distinct attributes of the 

 adult; that these attributes may or may not be inseparable 

 from one another, and that when they are separable they 

 may be transferred from one sex to the other, presumably 

 by the transference of factors in one part of a chromosome 



