72 ADAPTATION AND PROGRESS 



The so-called Mendelian law, the outgrowth of years of experi- 

 ment by Mendel in crossing garden peas of different varieties, 

 and verified since by hundreds of experiments on various plants 

 and animals by many botanists and zoologists, is thus stated and 

 illustrated by Professor Walter : 1 



When parents that are unlike with respect to any character are crossed, 

 the progeny of the first generation will apparently be like one of the parents 

 with respect to the character in question. The parent which impresses its 

 character upon the offspring in this manner is called the dominant. When, 

 however, the hybrid offspring of this first generation are in turn crossed with 

 each other, they will produce a mixed progeny, 25 per cent of which will be 

 like the dominant grandparent, 25 per cent like the other grandparent, and 

 50 per cent like the parents resembling the dominant grandparent. 



The law is represented by the following figure which shows that 

 in monohybrids the dominant character comes to the surface in 

 the second generation in three out of four of the offspring, one 

 of the three, however, being of pure breed, the other two being 

 hybrids. 



D (Dominant) R (Recessive) 



D(R) 



DD 2D(R) RR 



DD DD 2D(R) RR RR 



J, I I I L 



DD DD DD 2D(R) RR RR rr 



There are three principles involved in this law: (1) The exist- 

 ence of independent unit characters, (2) dominance, in cases where 

 the parents differ in a unit character, and (3) segregation of the 

 units contributed by the respective parents, this segregation being 

 found among the gametes formed by the offspring. 2 



1 Genetics, pp. 123 f. 



2 Castle, Heredity, p. 38; cf. Walter, op. cit., pp. 144, 145. 



