THE SCIENCE OF GENETICS 265 



naturally to group themselves is afforded by the law 

 of inheritance discovered by the Abbe Mendel about 

 the year 1865. This discovery has rendered possible 

 that rapid advance of the science of genetics, or the 

 study of the hereditary phenomena of organisms, 

 which has taken place during the first few years of the 

 twentieth century. There can be no sort of doubt that 

 Mendel's brief paper is the most important contribution 

 of its size which has ever been made to biological 

 science. Little apology is therefore needed for formu- 

 lating once again the law based by Professor Correns 

 upon the conclusions which this paper contains. 



Mendel's law relates to the inheritance of certain 

 definite characters, which have since been called allelo- 

 morphs. It is a distinctive feature of allelomorphic 

 characters that they are found to group themselves 

 naturally into pairs of more or less antagonistic 

 qualities. In many cases the pair is represented by 

 the presence and absence respectively of a certain 

 definite feature. The two allelomorphs of a pair may 

 be conveniently written as A and a. 



We have seen that the cells of zygotic organisms 

 organisms, that is to say, which have arisen by the 

 process of sexual reproduction contain a double 

 complement of hereditary qualities. Such cells may 

 contain A and A, a and a, or A and a. The forms A A 

 and aa are described as homozygotes, the form Aa as 

 a heterozygote. In the simpler cases we are enabled 

 to study the behaviour of such a single pair of allelo- 

 morphs by itself, without reference to any other features 

 which the animals or plants under consideration may 



