CHROMOSOMES AND MENDELIAN HEREDITY 287 



Examples of Mendelian Heredity. — Mendel crossed plants of a 

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

 bred dwarf race (% to 1^ feet in height) (Fig. 165). All the plants of the 

 first filial generation {F^) 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 {F^) comprised individuals of the two 

 grandparental types, tall and dwarf, in the proportion of 3:1. It was 

 further found that the tall individuals of this generation, though alike 

 in visible characters, were unlike in genetic constitution: one-third of 

 them, if bred 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 genera- 

 tion 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 {F^ produced nothing but dwarfs when inter- 

 bred: they 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. 



It should be understood that the above ratios merely indicate the 

 probability of obtaining the various types through chance combinations 

 of gametes. If the population is sufficiently large, the ratio is approached 

 rather closely; sometimes it is equalled exactly, even in a small population. 

 The ratios, then, represent a statistical result. 



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 2i factor, or gene, represented in Fig. 165 by 

 T) which tends to make the resulting plant tall. The germ cells of the 

 dwarf plant carry something {t) causing the dwarf condition. In the first 

 hybrid generation (Fi) both factors are present, T coming from one 

 parent and t from the other, but T "dominates" and prevents the 

 expression of the "recessive" t so that the plants of this generation are all 

 tall. When the hybrid (Fi) produces germ cells, the two factors for tall- 

 ness and dwarfness segregate, half of the gametes receiving T and the 

 other half t. Each gamete therefore carries either one or the other of the 

 two factors in question but never both; it is "pure" either for T or for t. 

 This segregation in the germ cells of factors associated throughout the 

 soma is the central feature of the entire series of Mendelian phenomena 

 and is often referred to as Mendel's first law. Since the gametes, both 

 male and female, produced by the hybrid plants of the Fi generation are of 

 two kinds (half of them bearing T and half bearing t), four combinations 



Mendel's Principles of Hetxdity and the first two editions of Castle's Genetics and 

 Eugenics. 



