48 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



the inheritance of their size characters or general body size must be 

 interpreted cautiously. Most cases of size-inheritance in both plants 

 and animals are complex and require a special interpretation, which is, 

 naturally enough at this period, Mendelian in nature. 



Throughout a number of recent papers on size-inheritance, there has 

 been, in the" main, one mode of explanation. Briefly stated, this expla- 

 nation hypothecates a number of size-determining factors, the accumu- 

 lative effect of which adds increments of size to the recessive small type. 

 It is assumed that no one of the factors is completely dominant. In 

 other words, size is thought to be due to multiple factors with incom- 

 plete dominance. Such a hypothesis must be carefully distinguished 

 from the cases involving multiple factors for characters with complete 

 dominance. To make the distinction clear, let us make use of a 

 hypothetical case involving multiple factors for one character, such that 

 one parent is homozygous for two factors, Ai and A2, while the other 

 parent lacks both of these. Using the ordinary Mendelian notation, 

 the cross is as follows : 



Ai Ai A2 A2 X ai ai a2 a2 Pi zygotes. 



Ai A2 + Ai A2 Pi gametes. 



ai a2 + ai a2 Pi gametes. 



Ai ai A2 a2 4- Ai ai A2 a2 Fi zygotes. 



Ai A2 + Ai a2 + ai A2 + ai a2 Fi gametes. 



Ai A2 + Ai a2 + ai A2 + ai a2 Fi gametes. 



IA1A1A2A2+ 2AiAiA2a2+ lAiAia2a2+ 2Aiaia2a2+ Iaiaia2a2 1 



2AiaiA2A2 laiaiA2A2 2aiaiA2a2 I F2 zygotes. 



4AiaiA2a2 



1(4 D) + 4 (3D) + 6 (2D) + 4(D) + 1(d) 

 or 15 dominants : 1 recessive. 



Now if, on the one hand, this illustrates a case involving two factors 

 for a character, with complete dominance, then the Fi generation appears 

 like the dominant parent, and the F2 zygotes consist of 15 dominants : 1 

 recessive. The first four classes in the F2 generation contain from one 

 to four doses of a dominant factor for the character; hence with com- 

 plete dominance they are like the Fi generation and the dominant 

 parent. The heterozygous condition of such completely dominant 

 factors can not be distinguished from the homozygous. The F2 genera- 

 tion has split up into 4" — 1 dominants : 1 recessive, n being equal to 

 the number of allelomorphic pairs. The class A1A1A2A2 has altogether 

 four doses of a dominant factor, but since one dose of these factors is 

 completely dominant, it is put in the same visible class as Aiaia2a2 or 

 aiaiA2a2. The F3 generation should demonstrate the existence of the 

 different kinds of F2 zygotes, which from external appearance are 

 grouped together as 15 dominants. Actual cases of this kind have 

 been demonstrated by Nillson-Ehle (1909, 1911), and East and Hayes 

 (1911). 



