248 HEREDITY AND ENVIRONMENT 



Some progress has been made, as was 

 pointed out in the last lecture, in identifying 

 certain structures of the germ cells with cer- 

 tain hereditary units, but quite irrespective of 

 what these units may be and where they may 

 be located it is possible, by means of the Men- 

 delian theory of segregation of units in the 

 germ cells and of chance combinations of these 

 in fertilization to predict the number of geno- 

 types and phenotypes which may be expected 

 as the result of a given cross. 



4. Presence and Absence Hypothesis. 

 Mendel spoke of the presence of contrasting 

 or differentiating characters in the plants 

 which he crossed, such as round or wrinkled 

 seeds, tall or short stems, etc. Many other 

 writers regard these contrasting characters as 

 positive and negative expression of a single 

 character, and consequently they speak of the 

 presence or absence of single characters: thus 

 round seeds are due to the presence of a factor 

 for roundness (A) while wrinkled seeds are 

 characterized by the absence of that factor 

 (a). Round seeds are wrinkled seeds plus the 

 factor for roundness. Most of the phenomena 



