370 ^^^- Bateson. 



Jii all cases of allelomorphism j-et sufficientl}' investigated, it has 

 been found that the heterozj'gote produces equal numbers of gametes 

 bearing each of the two allelomorphs. 



In a great number of instances the character of F^ is not inter- 

 mediate between the characters of the two i)arents, but resembles 

 one of them so much that F, is almost or quite indistinguishable 

 from that pure type. Such characters which appear, to the practical 

 concealment of the other allelomorph, were called by Mendel „domi- 

 nant", the excluded character being ..recessive". In our diagi'am black 

 is supposed to be dominant, and this fact is represented by placing the 

 black piece above the white. 



From these facts several simple conclusions follow, which are re- 

 presented in the diagrams. Thus Eli x DD gives all DR. which if 

 dominance be complete, may pass for pure D. Consequently DR x DR 

 seems to give on an average 3 D : 1 R (Fig. 1). 



In reality, of the H D two are imjjure on an average, and one 

 is pure. Similarly DR x RR gives an equal number of DR and RR 

 (Fig. 2), while I)K x DD appears to give all D (Fig. 3). 



The most striking consequence of allelomorphism is the paradox 

 that pure individuals may be bred from impure ones. Whenever an 

 R germ meets an R germ in fertilisation, the individual is pure R, 

 and the same is true for the D germs. (Consequently when an R 

 individual appears in the family, the D element is thrown out of that 

 individual for good and all. 



Avery simple illustration is that of the tall and dwarf („Cupid") 

 Sweet Peas (Luthyrus odoratus). ('rossed together these types give 

 the first filial generation, F, as we call it, all tall. Tallness is thus 

 dominant. "When Fj forms its germ-cells the characters tallness and 

 dwarfness segregate. 



By self-fertilisation the second generation — F., — is produced, 

 consisting on an average of three tails to one dwarf. The dwarfs 

 are then pure dwarfs, giving no more tails in their posterity: but of 

 the tails, one on an average will be pure tall; and two will be hetero- 

 zygous, giving the same mixture in the next generation. In the 

 photograph (Fig. 4) members of the F^ generation are shown. All 

 came from one pod of seed. The dwarfs are now pure dwarfs, though 

 they are the offspring of a tall plant and though one grandparent 

 was also tall. But of the tails some are pure tall, and some are 

 hybrids of tall and dwarf again, the pure being to the impure on an 

 average 1 : 2. 



Inheritance according to these simple rules has now been wit- 

 nessed in a great variety of characters among plants and animals. 

 The following are examples. In each case the dominant character is 

 put first. 



