346 EXPERIMENTAL STUDY OF INHERITANCE 



Thus, Mendel assumed that in the hybrid D(R) between a 

 parent with a dominant character D and a parent with a homo- 

 logous recessive character R the germ -cells segregate into two 

 camps, one half containing the dominant character inpotentia (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 f 50 with (d) 50 with (d) \ D(R) produces 100 



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



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

 (2)25 (d) (r) = 2 5 . (dr). 



(3)25 (r) (d) = 2 5 (dr). 



(4)25 (r) (r) = 2 5 (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 in- 

 dividuals in which two like characters have met together (the 

 pure dominants 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 find full expression in an individual development. The 

 contrasted characters to which the factors give rise are called 

 "allelomorphs," and 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 



