INDEPENDENT MENDELIAN INHERITANCE 69 



When the progeny of these tall hybrid plants were grown three-fourths 

 of the plants were tall, like the original tall variety, and one-fourth were 

 dwarf, like the original dwarf variety. Although like the tall plant in 

 appearance, therefore, the tall hybrid plants which were produced by 

 crossing a tall and a dwarf plant displayed their hybrid nature in the 

 kind of progeny they produced. To distinguish them from the tall 

 parents which produced only tall plants, they are accordingly called 

 tall hybrids. Continuing this experiment, Mendel found that the dwarf 

 segregants of the second generation bred true, they produced only 

 dwarf plants; but of the tall plants one-third only bred true, and the 

 other two-thirds proved to be tall hybrids, for three-fourths of their 

 progeny were tall plants and one-fourth dwarfs. The progeny of the 

 original tall hybrid plants, therefore, when subjected to this analysis 

 was found to consist of 1 tall : 2 tall hybrid : 1 dwarf. The experimental 

 results of the hybridization of tall and dwarf peas may accordingly be 

 diagrammed as in Fig. 33. 



FIG. 33. Results of hybridization of tall and dwarf peas. 



Mendel studied hybrids involving several different pairs of contrasted 

 characters and found that in every case one member of each pair of 

 characters was expressed unchanged in the hybrids, whereas the other 

 member of the pair became latent and its presence could be detected only 

 by growing the progeny of the hybrid. Those characters which were 

 expressed unchanged in the hybrid Mendel termed dominant, the latent 

 characters he called recessive. In the above experiment, for example, 

 tallness was dominant and dwarfness, recessive. Mendel saw that the 

 dominant character, therefore, in these experiments possessed a double 

 significance, that of parental character in which case a uniform progeny 

 of dominants is produced and that of a hybrid character in which case 

 one-fourth of the offspring display the contrasted recessive character. 

 In the above experiment the parental dominants are the tall parents and 

 the hybrid dominants are the tall hybrids. The condition of dominance 

 for a character, therefore, is determined by the fact that in the hybrid 

 that character is expressed to the exclusion of its contrasted character. 

 Dominance is by no means a universal phenomenon, but in Mendel's 



