DOCTRINE OF EVOLUTION AND RELATED IDEAS 499 



Close-breeding the tails showed them to be of two kinds: 

 (i) one-third of them (that is, one-fourth of the whole offspring) 

 were just as pure as the dwarfs were, and when self bred con- 

 tinued to produce tall forms indefinitely. Two-thirds of the 

 tails (one-half the total offspring) when inbred had three tall 

 offspring to one dwarf, — just as had been true of the first hybrid 

 generation. Further inbreeding of the hybrid offspring con- 

 tinues to repeat these proportions indefinitely: 1:2: i, — that is 

 one pure tall, two hybrid tails, and one pure dwarf. 



Mendel also found that yellowness of seed coat was dominant 

 over greenness; that smoothness of seed was dominant and 

 wrinkledness was recessive. He also found that these different 

 qualities were inherited absolutely separately; that is to say 

 that the yellow or green and the smooth or wrinkled quality 

 of the seeds might be transmitted equally in connection with 

 either the dwarf or the tall. 



The restdts stated in general terms might be expressed as 

 follows : When pure bred parents differ by the presence and ab- 

 sence of a certain heritable quality, all the offspring of the first 

 generation will be like one of the parents in bodily appearance. 

 This parent and its quality are said to be dominant. When the 

 individuals of this first hybrid generation are self-fertilized, or 

 crossed with each other, 25 per cent, will be pure and like the 

 recessive grandparent; 25 per cent, will be pure and like the 

 dominant grandparent; and 50 per cent, will be hybrids like the 

 immediate parents, having the bodily appearance of the domi- 

 nant grandparent. 



The diagram (Fig. 253) will aid the student in following the 

 general results.; 



493 . Mendel's Laws. — Three laws or principles were deduced 

 by Mendel from his experiments, and have been modified by 

 later workers. 



a. The principle of unit characters. This suggests that 

 organisms do not inherit the whole parental nature .as a unit, 

 but that each parental quality acts as a unit in inheritance. 

 Each individual, from the point of view of inheritance, is made 

 up of many independent unit characters which may be inherited 

 in any combination. 



