THE HEREDITARY BASIS 



15 



evident impress on much of recent thinking on the doctrine of 

 evolution. 



The discovery which has meant most for the progress of ge- 

 netics is unquestionably Mendel's law. The product of years of 

 research in the garden of the monastery at Briinn, Austria, the 

 principles enunciated by Mendel, owing to the fact that they were 

 published in a little-known journal, The Proceedings of the 

 Natural History Society at Briinn, failed to attract the attention 

 of the scientific world until they were made known independently 

 by three investigators, Tschermak, Correns and De Vries in the 

 year 1900. Thus began, with the beginning of the 20th century, 

 a new era in the study of genetics. Progress in this field since 1900 

 has taken' place at a very rapid rate. The amount of Hterature 

 devoted to the subject suddenly swelled to several times its 

 previous volume, and it is probably no exaggeration to say that 

 since the rediscovery of Mendel's law a greater advance has been 

 made toward a scientific analysis of the phenomena of heredity 

 than had been made during all preceding time. 



Mendel's law embraces two principles designated commonly 

 as (i) the law of dominance, and (2) the law of segregation. Ac- 

 .cording to the first, when two related but contrasted characters 

 are brought together in a cross the one appears to the exclusion of 

 the other, Mendel found, for instance, that when he crossed tall 

 and dwarf peas the immediate progeny were all tall instead of 

 intermediate in height. When he crossed green and yellow peas 

 the first generation (called the first fihal or Fi generation) con- 

 sisted entirely of yellow peas. The characters tall and yellow 

 are designated dominant in contrast to dwarf and green which 

 are called recessive. 



The recessive characters are not lost, as is shown when the 

 members of the Fi generation are either interbred or self-polli- 

 nated. They appear in the second or F2 generation along with a 

 certain proportion of dominants. Numerous experiments have 

 shown that in typical cases the dominant and recessive characters 

 are segregated in the second generation in the proportion of three 

 dominant to one recessive. The separation of the original char- 



