554 HEREDITY 



from that of either parent. A similar situation has been dis- 

 covered in connection with the breeding of Sweet Peas. Cer- 

 tain white-flowered varieties of Sweet Peas when crossed produce 

 red-flowered offspring. There are still a number of other situa- 

 tions that Mendel did meet in his experiments. 



To more recent investigators we are also indebted for some 

 present conceptions of the inheritable constitution of organisms. 

 Johannsen of Copenhagen, Denmark, who is responsible for the 

 pure line theory, has done much to establish the theory that 

 inheritance is due to the reappearance of the same organization 

 of protoplasm with reference to genes or character units in suc- 

 cessive generations and not to the transmission of external 

 characters. The sum total of all the genes in a gamete or fertil- 

 ized egg Johannsen calls a genotype, while an organism consid- 

 ered as to its appearance he calls a phenotype. Organisms may 

 be alike genotypically, that is, alike as to genes but be very 

 different phenotypically. For example, two Corn plants may 

 be exactly alike in their genes for size, yield, etc., but due to a 

 difference in environment differ greatly in these features. On 

 the other hand, organisms may be different genotypically but 

 be very similar phenotypically. 



Segregation and the Reduction Division. — • As previously 

 stated, the purity of gametes and the segregation of characters 

 depend upon the separation of the genes for contrasting char- 

 acters. A plant that is a hybrid for tallness and dwarfness can 

 not have gametes pure for tallness and dwarfness, unless the 

 genes for these contrasting characters are separated so as to 

 appear in different cells. The reduction division, which always 

 precedes the formation of gametes in both plants and animals, 

 affords a mechanism by which genes may be segregated. The 

 constant occurrence of the reduction division and also the fact 

 that it is the division in which chromosomes are separated, 

 suggest that it has some vital connection with heredity. 



It is generally believed that the genes are associated with the 

 chromatin of the nucleus and are, therefore, distributed with the 

 chromosomes to new cells during cell division. The chromatin 

 of a plant or animal consists of the chromatin contributed by 

 each of its parents. At each cell division this chromatin is 

 organized into a definite number of chromosomes, and there is 

 considerable evidence that the chromatin of each of the parents 



