BY RAYMOND PEARL 47 



prolonging incestuous mating one may in theory come indefinitely close to 

 reproductive purity, but in practise even this is extremely difficult, if not 

 impossible of accomplishment on any large scale or through any long period 

 of time. The fact simply is that a "pure line" in the strict sense of Johann- 

 sen 15 can not by definition exist in an organism reproducing as the domestic 

 fowl does. This, however, by no means indicates that the inheritance of 

 fecundity does not rest on a genotype basis, or, in other words, that fowls 

 do not carry definite genes for definite degrees of fecundity. 



We touch here upon an important point; namely, the relation of the 

 mode of reproduction to the mode of inheritance. As one reflects upon the 

 matter it becomes clear that it is only in the sense of a reproductive line that 

 we can not, by definition, have pure lines in organisms where the sexes are 

 separate. It is perfectly possible to have a line of such organisms in which 

 all the individuals are gametically pure with reference to any particular 

 character. For example, it is the simplest of matters to establish a line of 

 horses pure in respect to chestnut coat color. Any individual in such a line 

 mated to any other will never produce anything but chestnut offspring. So 

 similarly with any other character, it is only necessary to obtain homozy- 

 gous individuals in respect to any character in order to form a gametically 

 pure strain with reference to that character. 



It must further be kept clearly in mind that a reproductive "pure line" 

 (in the sense of Johannsen's definition) may be made up of individuals not 

 gametically pure (i. e.), homozygous. Thus suppose one crosses a yellow 

 and a green pea and then takes an F 2 heterozygote individual seed which 

 originated from a self -fertilized F T individual as the "single, self -fertilized 

 individual" with which to start a line. The individual which starts such a 

 line arose by self-fertilization and is selfed to produce progeny and would 

 thus fulfil every requirement of a reproductive "pure line" as defined by 

 Johannsen. Yet it would produce both yellow and green offspring. On 

 the other hand, as already pointed out, a line which is not, and from the 

 nature of its mode of reproduction never can be, reproductively "pure" 

 may be gametically so (i. e., have none but homozygous individuals with 

 respect to any character) . 



We then see that the fact that in fowls the sexes are separate and we 

 therefore can not have reproductive "pure lines" gives, per se, no reason to 

 suppose that fecundity is not inherited on a genotypic basis. We have 

 to consider the problem of genetic or gametic purity. Do we have homozy- 



15 Johannsen's definition is as follows: "Mit einer reinen Linie bezeichne ich 

 Individuen, welche von einen einzelnen selbstbefruchtenden Individuum abstammen." 

 ("Ueber Erblichkeit in Populationen und reinen Linien," p. 9.) 



