70 



The Journal of Heredity 



The Coefficients of Variability for the Monthly Weights of the Fi and F-i Generations of the 

 Berkshire X Duroc-Jersey Cross 



■5i% 



1 2 



Age in Months 



Graph of the coefficient of variability of the Fi generation. 



Smoothed curve for the C. V. of Fi. 



Graph of the coefiticients of variability of the F2 generation. 



Smoothed curve for the C.V. of Fa, (Fig. 19.) 



what was to be expected from the 

 genetic standpoint and it is in agree- 

 ment with the experience of practical 

 breeders. It is definite proof of two 

 things: First, that some of the factors 

 which have the power to stimulate 

 growth are not identical in the Duroc- 

 Jersey and the Berkshire breeds; second, 

 that there is some degree of homozy- 

 gosis for these growth-stimulating fac- 

 tors within the limits of each breed. 



To what extent being purebred from 

 the standpoint of the breed registry 

 swiety indicates homozygosis for these 

 factors; whether the number of factors 

 involved <an be determined; whether 

 the differences in growth indicate dif- 

 ferences in the identity of these factors 

 or merely differences in the combijia- 

 tions; and whether these factors are 

 so linked in groups as to make recom- 

 bination of the desirable ones im- 

 possible or at least impracticable; these 



are all interesting questions which 

 these data raise but are too meager to 

 answer. 



Other interesting questions, not pri- 

 marily genetic, relate to the shape of 

 the curve. The maximum \arial)ility 

 seems to be reached at or shortly fol- 

 lowing weaning time and coincides with 

 the period of the lowest mortality. 

 Does this mean that the best time to 

 select breeding stock for size and early 

 maturity is when they are from three 

 to six months old? Of course it was 

 impractical to keep the hogs which were 

 not desired for breeding purposes until 

 they were two or three years old and 

 therefore the question is still left open 

 as to whether the hogs finally would 

 not have become more uniform in size; 

 that is, whether the differences were in 

 potential size or in earliness of matu- 

 rity. 



