48 FECUNDITY IN THE DOMESTIC FOWL 



gote 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 (homozy- 

 gote) lines with respect to definite degrees of fecundity is simple. Practi- 

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

 character studied is not expressed in the male, and (6) it is subject to a wide 

 fluctuating variability caused by environmental conditions. The question 

 as to the gametic constitution of the fecundity lines here discussed obviously 

 can not be answered 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: 



1. Probably no line yet obtained is absolutely pure gametically in 

 respect to fecundity. It represents a mixture of a greater or less number of 

 fecundity genes. 



2. Lines which breed reasonably true to a definite degree of fecundity 

 may in most cases be taken to be made up of individuals bearing a pre- 

 ponderant number of genes of the particular degree of fecundity to which 

 the line breeds true, so that in gametogenesis a great majority of the gametes 

 formed carry only these genes. They also carry some genes of higher, or 

 lower fecundity, or both kinds. When individuals of a definite (e. g. y 

 "high") line thus constituted are bred together the majority of the offspring 

 will, purely as a matter of chance, be produced by the union of two high 

 fecundity gametes. It is quite possible that with families of the size 

 obtained with poultry nearly or quite every individual produced in the line 

 for several successive generations may be of this kind. In the long run, 

 however, it is to be expected that a small number of "off" individuals will 

 appear in the line. These originate by the chance union of two low 

 fecundity genes, or by the union of a "high" gene with a "low" gene of 

 great potency (as in the case of D31, cf. p. 335). 



3. The degree to which such a line will breed true will depend upon 

 the proportion of genes of one type (or of very similar types) present. The 

 higher such proportion the less frequently will the "off" individual segregate 

 out. The practical goal to be worked towards is, of course, to obtain 

 several lines not closely related, but all made up only of individuals homozy- 

 gous with respect to either high or low or any other definite degree of 

 fecundity. 



