INDEPENDENT MENDELIAN 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 T'T and they consequently will produce only tall plants, whereas the 
other two-thirds are of the genetic constitution 7't and will display segrega- 
tion in the ratio 3 tall:1 dwarf. Thedwarfsare allof the genetic constitu- 
tion tt, 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 JT’ and ¢. The term is a survival 
of the early days of Mendelism when attention was focussed on the 
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