CYTOLOGY AND MEN DELI AN HEREDITY 109 



the multiplication of cells and nuclei by division; the presence of chromo- 

 somes and their equational division; the disjunction of homologous 

 pairs of chromosomes at meiosis; the fusion of parental nuclei in syngam>-; 

 the fact that one of the fusing gametes may be only a nucleus; the 

 presence of a set of chromosomes from each parent in all the offspring's 

 nuclei. 



The rise of cytogenetics as a modern branch of biological science began 

 with the opening of the twentieth century, when it was realized that the 

 phenomena of inheritance described by Mendel in 1865 and redisco^ ered 

 in 1900 could probably be explained on the basis of chromosome behavior. 

 That nuclear elements had a special role of this sort was advocated before 

 1900 by Weismann and others. Mendel's clear analysis of his data 

 and his interpretation of the results in terms of representative factors, 

 combined with definite findings regarding the chromosome cycle, made it 

 possible in this century for the first time to proceed effectively with an 

 experimental investigation of the whole problem. How intimate the 

 fusion of cytology and genetics has become will be evident in the pages 

 to follow. 



Examples of Mendelian Heredity. — We may begin with one of 

 Mendel's own classic experiments with garden peas (Fig. 122). Plants 

 of a pure-bred race of tall peas (6 to 7 feet in height) and plants of a 

 pure-bred dwarf race (^/^ to 13^ feet in height) were selected for parents 

 (Pi). When the two types were crossed, all the plants of the first filial 

 generation (Fi) were tall like one of their parents. When these tall 

 hybrids were self-pollinated (or bred to one another), it was found that 

 the second generation (Fo) comprised individuals of the two grand- 

 parental types, tall and dwarf, in the ratio of 3 : 1 . It was further found 

 that the tall individuals of this generation, though alike in visible char- 

 acters, were unlike in genetical constitution: one-third of them, if bred 

 among themselves for another generation, produced nothing but tall 

 offspring, showing that they were pure for the character of tallness; 

 whereas, the other two-thirds, if similarly bred, produced again in the 

 next generation both tall and dwarf plants in the ratio of 3:1, showing 

 that they were hybrids with respect to tallness and dwarfness. The 

 dwarf plants of the second generation (Fo) produced nothing but dwarfs 

 Avhen selfed: thej^ were pure for dwarfness. From these facts it was 

 evident that the plants of the F2 generation, although they formed 

 only two visibly distinct classes, were in reality of three kinds: pure tall 

 individuals, tall hybrids, and pure dwarfs, these kinds occurring in the 

 ratio of 1:2:1. 



The explanation offered by Mendel for these phenomena may be 

 stated briefly as follows. The germ cells produced by the pure tall 

 plant carry something (now termed a factor, or gene, represented in Fig. 



