560 



GENETICS IN RELATION TO AGRICULTURE 



domestic fowl. The conclusions are based upon an analysis of data 

 obtained by trapnesting strains of pure-bred Barred Plymouth Rocks 

 and Cornish Indian Games, and F\ individuals and F 2 individuals ob- 

 tained by mating F\ individuals wter se and by mating them back to 

 their parents in all possible combinations. Over a thousand birds were 

 subjected to this definite experimental test. 



With respect to winter egg production hens naturally appear to fall 

 into three well-defined classes; (a) those birds which lay no eggs during 

 the winter period; (6) those which lay something less than about thirty 

 eggs; and finally (c) those which lay more than thirty eggs. Since egg 

 Jaying is a character strongly influenced by environmental conditions 



Per Cent Producing Indicated Number of Eggs or More 



FIG. 220. Contrasted flock curves of winter egg production of Barred Plymouth Rock 

 (solid line) and Cornish Indian Game (broken line) pullets. (After Pearl.) 



and somatic fluctuations, these classes are not absolute, nor on the other 

 hand are they by any means purely arbitrary as has been determined 

 by statistical studies of flock production during the winter period. The 

 differences which exist between the two breeds under investigation are 

 shown graphically in Fig. 220. Taking a production of thirty eggs or 

 more as the standard of comparison between the two breeds, it may be 

 seen from this graph that only about 6 per cent, of the total flock of 

 Cornish Indian Games produced as many eggs as this during the winter 

 period, whereas 54 per cent, of Plymouth Rock pullets measured up to 

 this standard of excellence. 



From a consideration of the data obtained from a wide series of 

 crosses, Pearl proposes the following analysis of the inheritance of 

 fecundity in fowls as measured by winter egg production. For the sake 

 of clearness and conformity to treatment in the remainder of the text, we 

 have used symbols different from those used by Pearl without, however, 

 in any way modifying the essential features of his analysis. 



