294 REPRODUCTION 



When the hybrid was crossed with the parent bearing the 

 wrinkled character, instead of being self-fertilised, the off- 

 spring consisted of round and wrinkled peas in equal pro- 

 portion, which is also what would be expected from Mendel's 

 hypothesis. 



Gametes of Gametes of 



Hybrid. Parent. 



R: —> W RW I RW 



= WW = to 



W RW I WW 



WW 



Mendel crossed peas varying in several other characters, and 

 obtained results similar to those described. For example, peas 

 having yellow cotyledons were crossed with varieties having 

 green cotyledons. Yellow was found to be dominant, but 

 segregation or splitting into the two types occurred in the 

 second generation : peas having green cotyledons appeared, 

 and in the proportion of r with green to 3 with yellow coty- 

 ledons. 



(iii) Characters which exclude or contrast with each other, as 

 the ' roundness ' and ' wrinkledness ' of peas, are spoken of as a 

 pair of allelomorphs. 



A plant or animal which arises from the union of two distinct 

 germ-cells is sometimes termed a zygote. 



The individual plant formed from the fertilisation of sexual 

 cells bearing similar allelomorphs is termed a hoinozygote (RR for 

 example). Where the allelomorphs are antagonistic the resulting 

 plant is spoken of as a heterozygote (as RW). 



(iv) The following have been found by experiment to behave 

 as allelomorphic pairs of characters : — 



