176 BREEDING 



set forth on p. 165. But it is introduced here to 

 pave the way for the sixteen-square Table which is 

 indispensable for explaining the 9:3:3:1 ratio. 



There are certain general terms and formulae 

 introduced by Mr. Bateson which have come into 

 general use. They may now be briefly indicated. 



The letter D is used to signify the dominant 

 character, and R the recessive, and consequently 

 the hybrid is spoken of as DR, the pure dominant 

 as DD, and the recessive as RR. It will be noted 

 that the single letters D and R are stated to signify 

 characters, whilst the organisms bearing these 

 characters are represented by two figures, DR, DD, 

 RR, etc. This latter fact is an expression of a part 

 of the Mendelian theory according to which the 

 organism is a double structure. Indeed, it may 

 almost be said that this is not a theory but a fact. 

 Every organism with which we are concerned in this 

 book is formed by the union of two germ cells. And 

 the simplest evidence of this duplicity is that the 

 number of chromosomes in the cells of an animal 

 or plant is twice that of the number in each of the 

 two germ cells which united to produce it. 



A single word for a germ cell is wanted. This is 

 supplied in gamete.'^ A more urgent need is a single 

 word for the result of the union of two gametes, the 

 animal or plant into which it develops. For this, 

 the word zygote'^ has been coined. 



In the illustration above, the single counter 



* From the Greek ya/xew, marry. 

 I From the Greek (vySv, a yoke. 



