INDEPENDENT MEN DELI AN INHERITANCE 71 



Since there is an equal chance for the occurrence of any one of these 

 types of combinations the progeny of a tall hybrid plant are in the ratio 3 

 tall : 1 dwarf. One-third of the tall plants are of the genetic constitu- 

 tion TT and they consequently will produce only tall plants, whereas the 

 other two-thirds are of the genetic constitution Tt and will display segrega- 

 tion in the ratio 3 tall : 1 dwarf. The dwarfs are all of the genetic constitu- 

 tion it, consequently they can produce only dwarf plants. The explana- 

 tion, therefore, satisfies all the requirements laid down by the experimental 

 results. 



Mendelian Terminology. As a result of the rapid development of 

 Mendelism during the past few years, a special terminology has grown 

 up which is used by practically all investigators in heredity. For those 

 terms which are in most common use, the following brief statements 

 are intended as interpretations of meanings and significance rather than 

 as mere definitions. 



The germinal representatives of Mendelian characters are variously 

 termed genes, factors, or determiners, three terms which are used synony- 

 mously in Mendelian literature. A Mendelian factor may be defined as 

 an independently inheritable element of the genotype by the presence 

 of which the development of some particular character in the organism 

 is made possible. The word gene was introduced by Johannsen to 

 designate an internal condition or element of the hereditary material upon 

 which some morphological or physiological condition of the organism is de- 

 pendent. These definitions do not hold rigidly as is always the case with 

 attempts to define something about which very little is known. Of the 

 terms, the term gene as introduced by Johannsen expressly denies any 

 assumptions as to the ultimate nature of the unit in question. The word 

 determiner on the other hand since it implies a rigid relation between an 

 hereditary unit and its end product, the character, is falling into disre- 

 pute, for very probably many hereditary units are concerned in the pro- 

 duction of all characters. The term factor, as applied to the units of 

 Mendelian heredity is perhaps more frequently used than any other and 

 is just as free from undesirable implications as to the nature of these 

 units on the one hand or their relation to the characters of the individual 

 on the other hand. It will consequently be used more frequently in this 

 book. 



Unit characters are those characters of the individual which behave 

 as units in heredity. Thus tallness and dwarfness in peas, since they 

 behave as units in heredity are called unit characters. To behave strictly 

 as units in heredity, character contrasts must depend on single factor 

 contrasts, as for example the character contrast of tall vs. dwarf in peas 

 depends upon a contrast of the factors T and t. The term is a survival 

 of the early days of Mendelism when attention was focussed on the 



