MENDELISM 435 



The two members of each of these pairs of character stand 

 to one another in the relation of dominant and recessive. In 

 the above list the dominant member of each pair is written 

 first. The two characters which form such a pair are called 

 allelomorphs ; and green is said to be allelomorphic to— i.e. forms 

 the other member of a pair with — yellow. Similarly yellow is 

 allelomorphic to green. 



We have so far, in considering the results of crossing an 

 animal or a plant, paid attention only to a single pair of 

 allelomorphs, tall and dwarf, black and splashed white. We 

 must now ask ourselves, What is the relation between character 

 belonging to different allelomorphic pairs ? And the answer, 

 broadly speaking, is that in the case of some characters there 

 is no relation, whilst in the case of others there is ; or, to amplify 

 the answer, in the case of some characters the members of distinct 

 allelomorphic pairs exercise a very definite effect on one another, 

 whilst in the case of others they are entirely independent of 

 one another. We will deal with the latter case first, because 

 it is simpler. 



What the result of mating a green round with a yellow 

 wrinkled pea will be, can be answered by reference to the table 

 of characteristics on p. 434, in which (it will be remembered) the 

 dominant member of a pair is written first. The result will be 

 a yellow round one. What the result of allowing these to self- 

 fertilise will be can be predicted from the result of making 

 drawings of two counters from each of two hats, which con- 

 tain equal numbers of green and yellow, red and white 

 counters. 



The number and nature of the possible combinations is given 

 by the number and contents of the squares in the Table on p. 436. 



Y and G stand both for yellow and green counters and 

 "yellow" and "green" germ-cells; R and W stand both for 

 red and white counters and "round" and "wrinkled" germ- 

 cells. The four pairs of letters along the top of the table 

 represent the four kinds of male gametes ; the pairs along the 

 side the four kinds of female ones ; the squares represent 

 zygotes. Let us consider the results of their union, beginning 

 at the top left-hand square. (In the following account Y is to 

 be read as yellow, G as green, R as round, and W as wrinkled.) 

 Here we have to do with union between two Y R gametes. 

 The result is a pure Y R zygote. In the square to the right of 



