IV 



PRESENCE AND ABSENCE THEORY 



37 



gether of such F x birds means the bringing together of two 

 such series of gametes. When this happens an ovum of 

 any one of the four types has an equal chance of being 

 fertilised by a spermatozoon of any one of the four types. 

 A convenient and simple method of demonstrating what 

 happens under such circumstances is the method some- 

 times termed the " chessboard" method. For two series 

 each consisting of four different types of gamete we re- 

 quire a square divided up into 16 parts. The four terms 

 of the gametic series are first written horizontally across 

 the four sets of four squares, so that the series is repeated 

 four times. It is then 

 written vertically four 

 times, care being 

 taken to keep to the 

 same order. In this 

 simple mechanical 

 way all the possible 

 combinations are 'rep- 

 resented and in their 

 proper proportions. 

 Fig. 5 shows the re- 

 sult of applying this 

 method to our series 

 RP, Rp, rP, 



FIG. 5. 



Diagram to illustrate the nature of the F 2 generation 

 and from the cross of rose comb x pea comb. 



the 1 6 squares represent the different kinds of zygotes 

 formed and the proportions in which they occur. As 



