SEGREGATION OF GERM-CELLS 347 



camps, one half containing the dominant character in potentia (d), 

 and the other half containing the recessive character (r). This 

 occurs in both males and females, so that when inbreeding 

 takes place the possibilities are expressible thus : 



D(R) produces (50 with (d) 50 with (d)\ D(R) produces 100 



100 egg-cells \ 50 with (r) 50 with (r)J sperm-cells 



(1) 25 egg-cells (d) fertilised by 25 sperm-cells (d) = 25 fertilised gametes (d). 

 (2)25 „ (d) „ „ „ (r)=25 „ „ (dr). 



(3)25 „ (r) „ „ „ (d)=2S „ „ (dr). 



(4) 25 „ (r) „ „ „ (r) =25 „ „ (r). 



To sum up, 25 (d) developing into 25 pure D. 

 50 (dr) „ „ 50 D(R). 



25 (r) „ „ 25 pure R. 



Bateson has proposed the useful term homozygous for individuals 

 in which two like characters have met together (the pure do- 

 minants and pure recessives), and heterozygous for individuals 

 in which unlike characters have met (the impure dominants). 



The Presence and Absence Theory.— One of the root-ideas of 

 Mendelism is that the inheritance includes numerous distinct and 

 independently heritable unit-factors. In certain cases Mendel 

 found that these factors occurred in contrasted or alternative 

 pairs, of such a nature that only one member of any one pair 

 can be carried by a germ-cell. The contrasted characters to 

 which the factors give rise are technically called " allelomorphs," 

 and, as we have seen, one is called dominant and the other 

 recessive. These can be distinguished by their behaviour in 

 breeding, but we do not know what the exact nature of the 

 contrast between dominance or recessiveness may be. 



In this connection Bateson has proposed a modification of the 

 Mendelian conception which may be called "the presence and 

 absence theory." "It is possible to express all Mendelian 

 phenomena in terms of a simpler system, according to which 

 the allelomorphism may be represented as consisting essentially 

 not in the presence of separate factors for the dominant and 

 for the recessive characters, but in the presence of something 

 constituting the dominant character which is absent from the 



