BREEDING POULTRY FOR EGG PRODUCTION. I7I 



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 (?'. e., homozy- 

 gous). Thus suppose one crosses a yellow and green pea and 

 then takes an F2 heterozygote individual seed which originated 

 from a self-fertilized Fi 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 pro- 

 duce progeny and would thus fulfil every requirement of a 

 reproductive "pure line" as defined by Johannsen. Yet it would 

 produce both yellow and green offsjDring. 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 indi- 

 viduals 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 homozygote lines in such cases 

 as those discussed in this paper? It plainly is the fact that one 

 can get lines of birds which, broadly speaking, will breed true 

 (perhaps throwing occasionally a few individuals not true to 

 the type of the line) to definite degrees of fecundity. 'The same 

 thing is true of milk production in dairy cattle, speed in race 

 horses, etc. What are these lines gametically? Theoretically 

 the formation of gametically pure (homozygote) lines with 

 respect to definite degrees of fecundity is simple. Practically 

 it is exceedingly difficult to do this, owing to the fact that (a) 

 the character studied is not expressed in the male, and ib^ it 

 is subject to a wide fluctuating variability caused by environ- 

 mental conditions. The question as to the gametic constitution 

 of the fecundity lines here discussed obviously can not be an- 

 swered finally now. • It is a matter for much further research. 

 One may, however, form a general conception of the probable 

 gametic constitution of such lines, which has much evidence 

 in its support. The essential points in such a conception are : 



