SEGREGATION AND THE REDUCTION DIVISION 555 



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. 



FIG. 483. A diagram illustrating the behavior of chromatin in the reduc- 

 tion division. _ For convenience the chromatin contributed by the father of 

 the plant, the division of whose cell the diagram illustrates, is shown black 

 and the chromatin contributed by the mother plant is shown white. In the 

 upper line, organization of the chromosomes and their pairing, each pair con- 

 sisting of one father and one mother chromosome; in the lower line, the dis- 

 tribution of the chromosomes in the formation of the daughter nuclei. In 

 this case one of the daughter nuclei receives one father and three mother 

 chromosomes, while the other daughter nucleus receives one mother and three 

 father chromosomes, but this is only one of a number of ways of distributing 

 the chromosomes. 



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 

 of the plant or animal whose cell is dividing organizes separately 

 into chromosomes, thus one-half of the number of chromsomes 



